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I burnt the cap to the letter n & lost the plastic bubble which went under the cap. It still works, but can I replace one key top?

Sadly the switch blister underneath is not replaceable on its own, the keycap and the scissor mech is, you’ll need to order it from here: MacBook Air - Keycaps. If you need to replace the entire keyboard assembly here’s that part: MacBook Air 11" (Mid 2011-Early 2015) Keyboard and here’s how to get to it MacBook Air 11" Mid 2013 Upper Case Replacement