You have two profiles within the Perifit Care app:
your general Perifit profile
your Community profile
You can change the username for your Community profile but not for your Perifit profile. However, no one in the app sees the latter.
Perifit account
When you first sign up in the Perifit Care app, you are required to provide your first and last name. This is the name that will be used for your Perifit account.
It cannot be changed but other users will never see your name. In challenger games, the app uses randomly selected names in order to protect the privacy of our users.
Community
Once your Perifit account is set up, you have the possibility to sign up for the Community under Settings > Our community. In there, you have the option to choose and change your username.
By default, it will be the name provided when creating your Perifit account but it can be changed by going to Profile (your photo / colourful circle in the top right corner) > Your profile > Edit profile > Name.
You can choose the name that will be displayed in the Community when posting, liking and commenting.