Hi,
I have prior experience in modifying IPTV apks.
However, I would like to know about the app you are willing to modify and the modifications you want, so I can decide whether the task is possible or not.
But let me just let you know it's not legal to modify the APK unless you own the source code. To avoid this, I can develop a brand new app for you within a reasonable time and cost so you can distribute to your customers without any fear.
Please contact so we can discuss further