Unfortunately, you won't be able to change your username often in Pokémon GO. According to the help page, you can only alter your username "a limited number of times," meaning you'll want to make your decisions wisely.

The website does not provide the exact number of times that you're capable of changing your username, but users have reported that it's no more than a handful of times. If you want to change your username, make sure you're confident in your new choice before committing.

You'll know that you've run out of username changes if the "Change Nickname" option isn't available to you in your settings.

If you want to change your nickname in the game and you've already used your allotted number of nickname changes, you'll have to submit a report to Niantic to have it changed. Unfortunately, there is no guarantee that the company will let you continue to change your username, depending on how many requests you've submitted, so be sure to make your username changes wisely.