MacBook Air m3 storage upgrade

You can custom-order a larger internal SSD – but cannot upgrade the internal SSD, once purchased.

You can add external storage space, and there are some external SSDs that are very portable. (I have one that is about the size of a credit card, and the thickness of an iPhone, that I use to back up a desktop Mac.). Obviously, if you need the extra storage space when you're "on the go", even a small external drive would be less convenient than a larger internal one.

ZL_24 wrote:

If I purchase a MacBook Air M3 15” screen with the 256GB SSD storage now, can I later upgrade it to the 512GB SSD? Or is that unchangeable once purchased?

You can not change the SSD once purchased.

Whatever you decide about the size of the internal drive, be sure to get at least one external drive for use in backing up your data.

The Mac comes with a backup utility (Time Machine), and others are available (Carbon Copy Cloner, SuperDuper!). But none of them can help you unless you get backup drives and make backups before catastrophe happens.