You can easily downgrade your Nord N10 by using the EDL flasher tool. First, you need to download the latest oxygen os firmware for your phone. Second, you need an EDL flasher tool like OPLUS EDL Tool or DeepSmartTool Unbrick Tool that supports free flashing.

Download the firmware and unzip it on your pc select the model from the tool. Select the raw programming file from the firmware folder. Turn off your phone and turn it into edl mode by holding volume up + down and insert the USB cable. Then connect into PC and start flashing with OPLUS EDL Tool or DeepSmartTool.

Download Nord N10 firmware - EDL firmware
Download OPLUS EDL Tool - OnePlus OPLUS EDL Tool
DeepSmartTool - Unbrick Tool for Qualcomm