Reset MacBook Air

See the following Apple Support article for information on resetting a MacBook Air (and note that step 2 only applies to recent models): What to do before you sell, give away, or trade in your Mac - Apple Support

Below page has other options/cases / details:

Erase your Mac and reset it to factory settings - Apple Support

your macbook could be this:

MacBook Air (11-inch, Early 2015) - Technical Specifications

or this:

MacBook Air (11-inch, Mid 2011) - Technical Specifications