Changing boot & shutdown screen on Lenovo Tab M10 FHD Plus

Can anyone help me with how to change the boot screen and shutdown screen on my Lenovo Tab M10 FHD Plus? It seems to me that I cannot change the files in the system. It won't let me edit them or replace them etc. I've tried for hours with this and got nowhere. I tried changing files in system files but they're locked up and can't be edited or overwritten, etc. I tried an app that's meant to write over them for you with new animation files for start-up and shutdown, but that won't do the trick either. Any help would be much appreciated.