First here’s how: [guide|38535] As for what to put in, I strongly recommend you stick with either a real Apple unit [https://beetstech.com/blog/apple-proprietary-ssd-ultimate-guide-to-specs-and-upgrades|The Ultimate Guide to Apple’s Proprietary SSDs] or one of the third party drives designed to fit and work in your system either from [https://eshop.macsales.com/shop/ssd/owc/macbook-air/2013-2014-2015|OWC] or [https://us.transcend-info.com/Products/No-957|Transcend].
I personally don’t recommend using M.2 adapters with M.2 SSD’s while they do appear to work they just don’t have the lifespan of the other SSD’s. These adapters and some SSD’s create heavy CRC errors which over time wear out the drive.