Can internal 128GB HDD on MacBook Air be upgraded?

What SUB-MODEL is the Macbook Air? That makes a critical different in the answer. That is quickly available by doing ""About this Mac...," from your Apple menu. it will include a year:

Why is it critical?

• If your Macbook Air is 2017 or older, that are options for upgrading storage.
• If 2018 or newer, there are NO options to upgrade. Starting with the 2018 models, storage became integral with the logic board.

Excellent! Then you have options. I love options.

I get my Mac upgrade hardware from this vendor,

I recommend the "kit" version because it includes tools plus a transfer enclosure that makes the installation and setup much easier. Also note farther down on the page the links to install videos (yours is the "7,2" version). This is an at-home job suitable to most average end users.

I would also consider the cheaper version with the 3-year warranty over the five-year version. Your computer is already an eight-year old model and you might as well save some money as the drive will not work internally in newer Macs.