I need to update my mac and want to back it up first. What is the best device to use for a 13-inch early 2015 model macbook pro?

Any USB external drive will work. I'd opt for a SSD over a HDD for speed and reliability.

What BobTheFisherman easy is completely accurate.

For Time machine, a drive at least twice the size of what you might want to back up is preferable for long term trouble-free operation. some say even larger, like 2.5 to 3x.

Time machine runs in the background at low priority. you should not pay a substantial premium for a drive that is FAST, because you will not see that extra speed in day-to-day use.