The RAM can't be replaced by conventional means on the MacBook Air, however this issue can sometimes be repaired.
Remove the board and check the RAM chips (group of black rectangle chips on front/back of board) for any liquid damage. If there is liquid around a particular chip, note which one is affected and try cleaning with 95%+ isopropyl. If not successful, the chip will have to be reflowed and you will need a decent hot air station to do this.
If no liquid is visible, this has likely been dropped and caused a solder ball under the RAM chip to crack. All the RAM chips will need to be reflowed in this case, since you do not know which one is affected.
If not successful, the affected chip(s) will need to be replaced. If you do not have the required equipment, take it to a competent repairer