First of all Hot Reload is still in preview as of Aug 15, 19' and is being tested by the community and has a bunch of known bugs and issues.
Below are the minimum requirements for you to be able to use hot reload
Visual Studio 2019 16.3 or greater
Visual Studio 2019 for Mac 8.3 or greater
Xamarin.Forms 4.1 or greater
Now you can enable hot reload by:
On Windows, XAML Hot Reload can be enabled by checking the Enable Xamarin Hot Reload checkbox at Tools > Options > Xamarin > Hot Reload.
On a Mac, XAML Hot Reload can be enabled by checking the Enable Xamarin Hot Reload checkbox at Visual Studio > Preferences > Projects > Xamarin Hot Reload.
Limitations (Known)
You can't add, remove, or rename files or NuGet packages during a XAML Hot Reload session.
If you add or remove a file or NuGet package, rebuild and redeploy your app to continue using XAML Hot Reload.
Set the linker to Link None for the best experience. The Link SDK only setting works most of the time, but it may fail in certain cases.
Debugging on a physical iPhone requires the interpreter to use XAML Hot Reload. Add --interpreter to the Additional mtouch arguments field in your iOS Build settings to use XAML Hot Reload.
Any references created by assigning a control to another field or property using its x:Name value won't be reloaded.
Updating the visual hierarchy of your Shell application in AppShell.xaml can cause issues maintaining the state of your application. Rebuild the app to continue reloading.
XAML Hot Reload can't reload C# code, including event handlers, custom controls, page code-behind, and additional classes.
For future references, you can check the official document of Hot Reload