I do not own the phone myself, but i believe i have an answer for you. Be careful tho and read through all the links before you flash your phone:
1. You need to unlock your bootloader. This might be the biggest hurdle as Nokia is not officially supporting unlocking the phone. You might have to look around for that
2. Follow the instructions here:
https://forum.xda-developers.com/no.../tool-stock-rom-flash-tool-nokia-6-1-t4023965
You will find a image of the stock firmware here:
https://fih-firmware.hikaricalyx.com/hmd_en.html
Be careful to select the correct file for your phone (TA-1045)
Althoug at this point you might even want to look at the custom roms for your phone as they might offer a better design and speed as the stock Android 10 while still having the latest patches.