Can I upgrade the RAM?
Can I upgrade the RAM in my Macbook Air (late 2015) from 4GB to 8GB?
Sadly No, The RAM is soldered to the logic board and is not upgradable. Your only choice here is to exchange the logic board with one that has 8 GB which is not economical. It would be cheaper to buy a new system.
Review this IFIXIT teardown: MacBook Air 13" Early 2015 Teardown. Jump to Step 7. The red outlined chips are the RAM chips.