Is it possible to change the WinUI3 msix display name after its installed..

This can be done. You also can do it if you just provide an update to the package. The only important thing is that the package identity stays the same, this means that the package name itself cannot change.

To give an example. Here is an example Package.appxmanifest file. This is not from a WinUI 3 project, but this is from a packaged desktop application. There is no difference since WinUI 3 applications desktop applications.

<?xml version="1.0" encoding="utf-8"?>

<Package
  xmlns="http://schemas.microsoft.com/appx/manifest/foundation/windows10"
  xmlns:mp="http://schemas.microsoft.com/appx/2014/phone/manifest"
  xmlns:uap="http://schemas.microsoft.com/appx/manifest/uap/windows10"
  xmlns:rescap="http://schemas.microsoft.com/appx/manifest/foundation/windows10/restrictedcapabilities"
  IgnorableNamespaces="mp uap rescap">

  <Identity Name="Untrusted.ConsoleTest" Publisher="CN=Darran" Version="1.0.1.0" />

  <mp:PhoneIdentity PhoneProductId="00000000-0000-0000-0000-000000000000" PhonePublisherId="00000000-0000-0000-0000-000000000000"/>

  <Properties>
    <DisplayName>ConsoleTest</DisplayName>
    <PublisherDisplayName>Darran</PublisherDisplayName>
    <Logo>Images\StoreLogo.png</Logo>
  </Properties>

  <Dependencies>
    <TargetDeviceFamily Name="Windows.Universal" MinVersion="10.0.0.0" MaxVersionTested="10.0.0.0" />
    <TargetDeviceFamily Name="Windows.Desktop" MinVersion="10.0.14393.0" MaxVersionTested="10.0.14393.0" />
  </Dependencies>

  <Resources>
    <Resource Language="x-generate"/>
  </Resources>

  <Applications>
    <Application Id="App"
      Executable="$targetnametoken$.exe"
      EntryPoint="$targetentrypoint$">
      <uap:VisualElements
        DisplayName="ConsoleTest"
        Description="ConsoleTest"
        BackgroundColor="transparent"
        Square150x150Logo="Images\Square150x150Logo.png"
        Square44x44Logo="Images\Square44x44Logo.png">
        <uap:DefaultTile Wide310x150Logo="Images\Wide310x150Logo.png" />
        <uap:SplashScreen Image="Images\SplashScreen.png" />
      </uap:VisualElements>
    </Application>
  </Applications>

  <Capabilities>
    <Capability Name="internetClient" />
    <rescap:Capability Name="runFullTrust" />
  </Capabilities>
</Package>

In particular, notice the DisplayName element for the Properties element, the DisplayName element for the VisualElements element and the Description element for the VisualElements.

The installer UI will show:

Start will show:

Now, if I update things so that the Package.appxmanifest is as follows:

<?xml version="1.0" encoding="utf-8"?>

<Package
  xmlns="http://schemas.microsoft.com/appx/manifest/foundation/windows10"
  xmlns:mp="http://schemas.microsoft.com/appx/2014/phone/manifest"
  xmlns:uap="http://schemas.microsoft.com/appx/manifest/uap/windows10"
  xmlns:rescap="http://schemas.microsoft.com/appx/manifest/foundation/windows10/restrictedcapabilities"
  IgnorableNamespaces="mp uap rescap">

  <Identity Name="Untrusted.ConsoleTest" Publisher="CN=Darran" Version="1.0.2.0" />

  <mp:PhoneIdentity PhoneProductId="00000000-0000-0000-0000-000000000000" PhonePublisherId="00000000-0000-0000-0000-000000000000"/>

  <Properties>
    <DisplayName>New ConsoleTest</DisplayName>
    <PublisherDisplayName>Darran</PublisherDisplayName>
    <Logo>Images\StoreLogo.png</Logo>
  </Properties>

  <Dependencies>
    <TargetDeviceFamily Name="Windows.Universal" MinVersion="10.0.0.0" MaxVersionTested="10.0.0.0" />
    <TargetDeviceFamily Name="Windows.Desktop" MinVersion="10.0.14393.0" MaxVersionTested="10.0.14393.0" />
  </Dependencies>

  <Resources>
    <Resource Language="x-generate"/>
  </Resources>

  <Applications>
    <Application Id="App"
      Executable="$targetnametoken$.exe"
      EntryPoint="$targetentrypoint$">
      <uap:VisualElements
        DisplayName="New ConsoleTest"
        Description="New ConsoleTest"
        BackgroundColor="transparent"
        Square150x150Logo="Images\Square150x150Logo.png"
        Square44x44Logo="Images\Square44x44Logo.png">
        <uap:DefaultTile Wide310x150Logo="Images\Wide310x150Logo.png" />
        <uap:SplashScreen Image="Images\SplashScreen.png" />
      </uap:VisualElements>
    </Application>
  </Applications>

  <Capabilities>
    <Capability Name="internetClient" />
    <rescap:Capability Name="runFullTrust" />
  </Capabilities>
</Package>

Note that the previously mentioned elements have been updated, but the Name attribute for the Identity element has not changed. If you also note the Version attribute of the Identity element, you will notice that this has change. It must change to provide a nice smooth upgrade.

Anyway, the installer UI will show:

Start will show:

So it is possible to change things like the name and description, but it is important to keep the identity the same if you want a smooth upgrade experience.