Did you purchase a 128gb or 256gb hard drive for your macbook air or macbook pro and now you want to get more storage? Pretty much the only way to reliably upgrade is to buy a used hard drive on the secondary market. You can buy new generic drives but I don’t know how reliable they are. You would have to do your research first, of course.
Changing your hard drive (solid state drive) on a newer macbook air or pro is pretty easy. My instructions are for a 2014 macbook air but the others that have this same kind of hard drive will be similar (or even identical). You need a #5 pentalobe (P5) (to open the bottom case) and a T5 torx (to remove the screw holding down the hard drive.