You can flash the SuperSU uninstaller .zip file using a custom recovery, such as TWRP. Once it's fully installed on your device it will remover SuperSU completely and make your device unrooted. So you can further install Magisk root.
If you want to know the whole process to remove SuperSU from your device then continue to the following full instructions below.
Visit this page; uninstall SuperSU if you wish to uninstall the SuperUser access management tool.
Your device should already have TWRP. If not then please head to how to install TWRP recovery.
How to remove SuperSU and Unroot Android device
- Then transfer the UPDATE-unSU.zip file to Android internal storage.
- Now enter your device to TWRP recovery mode.
- Tap the Install button from TWRP main menu to choose UPDATE-unSU.zip file then Swipe to Confirm Flashing the zip file.
- Once done, just perform Wipe Cache / Dalvik on TWRP.
- You can reboot your device now.
Finish now SuperSU is no longer on your device. You can continue to how to install Magisk root on Android of yours.