Create preview_upgrade_instructions.md (#2915)

docs/preview_upgrade_instructions.md

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+# Instructions for upgrading a WinUI 3 Preview 1 app to use WinUI 3 Preview 2
+
+While it's possible to upgrade your Preview 1 application to use the new Preview 2 bits, please be careful and follow all steps in order below when doing so. The steps below outline the process that the team had to take while updating their own Preview 1 apps. 
+
+If these steps don't work for you, feel free to [file an issue](https://github.com/microsoft/microsoft-ui-xaml/issues/new/choose). The easiest way to work around this is to just create a new Preview 2 project and copy your content over. 
+
+## Upgrading steps
+1. Make sure all Preview 2 prerequisites are installed. See [installation instructions here](aka.ms/winui3/preview2#install-winui-3.0-preview-2).
+2. Use the NuGet package manager (right-click on the project and select “Manage NuGet Packages…” from the context menu) 
+3. Select the “Microsoft.WinUI” package, ensure that “Include prerelease” is checked 
+4. Open the "Package source" dropdown in the top right corner and make sure that WinUI.Dependencies is selected. 
+5. Select the latest version of the package, and then click “Upgrade”, accepting prompts that appear 
+6. If your project is a C# “Blank App (UWP)”, C# “Class Library (UWP)”, C# “Windows Runtime Component (UWP)”, C++ “Blank App (UWP)”, or C++ “Windows Runtime Component (UWP)” project, then **no further changes are necessary.**
+7.  If your project is a C# “Class Library (WinUI in Desktop)” project:
+    - Open the .csproj and change <TargetFramework>netcoreapp5.0</TargetFramework> to <TargetFramework>net5.0</TargetFramework> 
+8. If your project is a C# “Blank App, Packaged (WinUI in Desktop)” project :
+    - Open the .csproj in the main app project and change <TargetFramework>netcoreapp5.0</TargetFramework> to <TargetFramework>net5.0</TargetFramework>  
+    - Open the .wapproj in the associated Windows Application Packaging Project and make the following edits: 
+        - Add the following line to the first `<PropertyGroup>` element after `<Import Project="$(WapProjPath)\Microsoft.DesktopBridge.props" />`:  
+
+        ```xml
+         <AppxTargetsLocation Condition="'$(AppxTargetsLocation)'==''">$(MSBuildThisFileDirectory)build\</AppxTargetsLocation> 
+         ```
+
+        - Change `<Import Project="build\Microsoft.WinUI.AppX.targets" />` to `<Import Project="$(AppxTargetsLocation)Microsoft.WinUI.AppX.targets" /> `
+
+        - Create a new C# “Blank App, Packaged (WinUI in Desktop)” project, locate the folder containing the associated Windows Application Packaging Project, and copy the contents of the `“build\”` subdirectory (`LiftedWinRTClassRegistrations.xml` and `Microsoft.WinUI.AppX.targets`) into the corresponding `“build\”` subdirectory of your app’s associated Windows Application Packaging Project, overwriting any existing files when prompted.
+9. If your project is a C++ “Blank App, Packaged (WinUI in Desktop)” project:
+    - Open the .wapproj in the associated Windows Application Packaging Project and make the following edits: 
+
+        - Add the following line to the first `<PropertyGroup>` element after `<Import Project="$(WapProjPath)\Microsoft.DesktopBridge.props" />`:  
+
+        ```xml
+        <AppxTargetsLocation Condition="'$(AppxTargetsLocation)'==''">$(MSBuildThisFileDirectory)build\</AppxTargetsLocation> 
+        ```
+        - Change `<Import Project="build\Microsoft.WinUI.AppX.targets" />` to `<Import Project="$(AppxTargetsLocation)Microsoft.WinUI.AppX.targets" />`
+
+        - Create a new C++ “Blank App, Packaged (WinUI in Desktop)” project, locate the folder containing the associated Windows Application Packaging Project, and copy the contents of the `“build\”` subdirectory (`LiftedWinRTClassRegistrations.xml` and `Microsoft.WinUI.AppX.targets`) into the corresponding `“build\”` subdirectory of your actual project’s associated Windows Application Packaging Project, overwriting any existing files when prompted 
+
+