How do I force reinstallation of an apk with different signature on rooted Android?
You can use an app called 'Lucky Patcher' witch contains many system tweaks, (root and busybox are required) then follow the following steps:


Go to 'Toolbox' tab in the buttom.
Select 'Patch to Android'.
Check 'Disable signature verification in the package manager'.
Press 'Apply' and wait for your phone to reboot.


and that's it. 'Lucky Patcher' also has an Xposed module to do the same task if Xposed is already installed in your system.