How to change the username (for multiplayer)

when i try to join my friends room, it says that the username is invalid, tho it has between 4 and 20 letters

In order for us to provide better support, we need to see the log generated by citra. This guide will walk you through how you can obtain the log file: How to Upload the Log File

In the multiplayer window, make sure to set your username in the top left part of it.