Hi @Marcos Feliciano ,
You're correct, PHP 7.4 for Linux is not yet available. Also, 7.4 and 7.3 are actually "major" versions. 7.3.31 would be considered a minor version (the .31). There should be an announcement made on the App Service Team Blog when it becomes available for Linux.
How to update your app to target a different version of PHP: The version of PHP provided by app service can be modified through the Azure Portal: Configuration>General Settings>Stack Settings>Minor Version
Hope that helps.
-Grace