PEI/Ultem replacement guide
If your PEI/Ultem sheet on MK42 heatbed is damaged, you can replace just the sheet instead of buying a whole new MK42 bed assembly. Please follow this guide.
Don't forget to rerun the calibration after you are done replacing the sheet. Proceed according to the handbook chapter 6.3.1. Calibration flow. Run the calibration even on a pre-assembled printer, since the geometry might have slightly changed.
During the process, you will need Limonene to dissolve the glue holding the sheet down. Here are some sources: