Besides the solution by Propeller, I would tell the method to do that by ADB (just for knowledge say)...
MIUI lockscreen is the ROM's built in lockscreen which you really dont want to uninstall. Simply removing the theme would do the job.
Boot your phone to recovery by pressing and holding volume up + power button (going into recovery isnt necessary. But recommended if you haven't enabled USB Debugging or didn't yet add that PC's RSA Fingerprint to phone in which case ADB wont work if your phone is Powered On. Otherwise there's no need to go into recovery).
Connect your phone to PC and run these commands for ADB:
adb shell
mount -o rw,remount /data
cp /data/system/theme/lockscreen /sdcard/
rm -r /data/system/theme/lockscreen
reboot
Or
If you have a recovery with built in terminal emulator such as TWRP, you dont even need to use ADB. Just boot your phone to recovery and go to Mounts
in recovery and make sure /data
is selected.
Then open terminal emulator inside your recovery and run these commands->
cp /data/system/theme/lockscreen /sdcard/
rm -r /data/system/theme/lockscreen
reboot
Both these methods will backup your lockscreen theme to sdcard and also remove it from system and your default lockscreen would be used at next restart.