Hi there, TanavFlies!
Thanks for posting in the Apple Support Communities regarding your App Store and iTunes Store nickname. We're happy to give you some information on how to change it!
When leaving a comment/review of an app, your nickname is displayed. This nickname is different than your full name listed on your Apple ID account. While you are not able to change the nickname from your iOS device, you can use a computer to do so.
If you're using a Mac, open the macOS App Store and click on your name at the bottom of the sidebar on the left. If prompted, sign into your Apple ID. Then, click "View Information" from the upper right of the account page. Once the next page loads, scroll down to the "Manage" section. Here you will see your nickname and have the option to edit it. Changing the nickname here will change how the nickname appears when leaving comments/reviews on apps.
If you are using a Windows computer, you can change your nickname by clicking Account > View My Account. If prompted, sign in with your Apple ID. Scroll to the "Settings" section and click the edit button next to your nickname: Set up and manage an iTunes Store account on PC - Apple Support.
Cheers.