I want to change my nickname in the App Store.

Insulting users in the Apple Support Communities, especially one who has taken their volunteer time here to attempt to assist you, is a violation of the User regulations you agreed to when you opened your account here

Having written only 5 reviews in the 16 years that I've participated in the Apple Support Communities I didn't realize that the system had created a nickname for me. So I didn't understand your question. As it appears the system used my Apple ID as my nickname!

As Raicya has stated, you can only change the nickname from your account details in the iTunes Store on a Mac (or PC). On a Mac open the Music app. Select iTunes Store in the left collum. Select Account on the right in Music Quick Links and you can manage your Nickname in the last section at the bottom of the page.

Some aspects of your account details can only be accessed and modified in iTunes on a Mac or PC.

iPad mini 5, iPadOS 16

I want to change my nickname in the App Store.

Do you have a macOS or Windows OS computer?

Just tested for you. It is not possible to change your Nickname in the App Store from an iPad.

It is possible from a computer.

Send feedback to Apple, see -> Product Feedback - Apple

Are you referring to monomial, your Apple ID?

You would need to speak directly to Apple Support about changing that.

World-wide Apple Support -

Contact Apple for support and service – Apple Support (UK)

M1 Mac mini, macOS 13.4;  Watch, series 7 GPS/cellular, watchOS 9.5; iPad Pro 2nd Gen, iPadOS 16.5; iPhone 12 Pro, iOS 16.5; TV 4th Gen, tvOS 16.5; HomePods, audioOS 16.5 and Airport Express w/ Airplay 2

The option to 'Manage' your Apple ID via iPad is less extensive than via a macOS or Windows OS computer.

Via a Windows OS computer users with an iPhone/iPad can download / buy such as ringtone, songs an others.

And also Manage their Apple ID and change their nickname.

Visit an Apple Store or an Apple Authorized Reseller in your region for assistance.

May be they can and will help you (I am not sure, never did it).

You will first have to make an appointment due to the pandemic.

See: Contact Apple for support and service

Contact Apple for support and service - Apple Support

See my previous post.