A Facebook connected account: will change the game profile picture and username accordingly to the one you have on Facebook.
An Apple ID connected account: please check the pop-up when linking your Game Center account to choose whether you'd like to share your name with fellow players.
Guest account: You cannot add or change a profile picture with this type of profile. The same applies for the username.
Nevertheless, you can change the name, the logo and the flag of your team.
1. Tap on "Equipment" from the main screen (left side)
2. To change the Logo, select the top right icon in order
To change the team name/abbreviation or flag, tap on the "pencil" icon
3. Write the new team abbreviation and/or Name on the left side (please make sure that special characters and emoji might not be fully supported - and please don't pick anything offensive to other players)
Scroll down and select your team country (be aware that changing the team country has no effect on the country leaderboard!)
Don't forget to Confirm otherwise the changes won't be saved!