How to change £ key back to # on MacBook Air?

FredBassett wrote:

My hash key now is a £ key how to revert it back to a #

Just go to system settings > keyboard > text input > edit > all input sources and use the minus button at the bottom to remove the item called "British". You should have just one item on the list, with the name "US" or "ABC".

what you are describing depends on which keyboard "input sources" is currently active.

one is likely British and the other may be U.S.

Except for the decorative key caps, the keyboard Hardware is generic. It does not know what glyphs are on your keys. it generates a key-position code only, and that gets looked up through your input source setting to produce letter and number and symbols.