READ FIRST: Help with apk-file (decompile, change version number, compile and sign)

Hello, I just had an faulty update in the PlayStore and we need to roll back to a previous version without having the project files available. So we need you to decompile the file, increase the version number and compile & sign the file again. I know this can be achieved with the software "apktool" but can't get it to run properly on my system.