PEI/Ultem replacement guide
If the smooth PEI layer on your heatbed or on your flexible steel sheet is damaged, instead of buying a whole new heatbed or a whole new steel sheet, you can purchase just a new spare PEI sticker (this one is for the MK2/S heatbed and this one for the magnetic sheet used on MK2.5 and MK3 or its newer versions). Make sure you are logged in, as this item belongs to the spare parts section.
Please follow this guide to replace your sheet.
Don't forget to run the calibration wizard again after you are done replacing the sheet.. 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: