How can I warm up a fleishige meal in a treif microwave, or in a kosher milky microwave?
Several authorities have written that a microwave can be kashered by ensuring that it is completely clean, leaving it unused for 24 hours, and then turning on the microwave with a cup of water inside for several minutes (until an amount of water has evaporated). Note that the tray on which food is placed would require hagala, or replacement with another tray.
The cup used for a meaty microwave should be meaty, and for a milky microwave, milky. If the microwave is (possibly) tereif, a disposable cup should be used, which is discarded after the kashering procedure.
If this procedure cannot be done, the food can be heated if it is hermetically sealed by two covers.