Uninstall SuperSu with binaries
If the SuperSU app to remove SuperSU binary doesn't work, you'll have to wipe and reinstall but fear not as this will not wipe your user data.

Make sure to have the cm rom for your device and google apps if needed.

Now that you have them, preferably in your device for easier access, boot your device into recovery.

In your custom recovery you should select the wipe option>advanced wipe options>select only system and wipe it.

After this you just flash CyanogenMod normally, and Gapps if needed, reboot your device and that should be all.

PD. Is always good to make a backup.