Hi @Stefanos Evangelou ,
You should be fine deploying your 4.6.1 app to ASP.NET 4.8 runtime app service instance as .NET v4.x are backwards compatible. Have a look at https://learn.microsoft.com/en-us/dotnet/framework/migration-guide/version-compatibility, but should you run into issues, https://learn.microsoft.com/en-us/dotnet/framework/migration-guide/retargeting/4.6.1-4.8 should help direct you to correcting any issues.