How do I back up my older MacBook Air with a sandisk

Horacon wrote:

How do I back up my older MacBook Air with a Sandisk?

You have not provided any context or details but typically one would use Time Machine to perform backups:

Back up your Mac with Time Machine - Apple Support

A Sandisk what? There are a multitude of items made by Sandisk including SSDs, USB Flash drives, SD Cards, Compact Flash card, etc.