PEI/Ultem sheet on MK42 heatbed can be easily replaced. Please follow this guide.
Don't forget to rerun the calibration following the Handbook chapter 6.3.1. Calibration flow. Follow the kit version even on preassembled version as the geometry might have slightly changed.
During the process you will need Limonene to dissolve the glue holding the sheet down. Here are few sources:
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