After downloading the pack, create a world.

Exit Minecraft and extract the contents of the pack to %USERPROFILE%\AppData\Local\Packages\Microsoft.MinecraftUWP_8wekyb3d8bbwe\LocalState\games\com.mojang\resource_packs

Open Minecraft and after clicking "Play Beta" open the world settings (the pencil icon to the right of the world button)

Open "resource packs" -> "my packs" and activate the RTX pack you installed.

Have fun