1.Go to this website: https://truckersmp.com/profile/settings , scroll down, until you see big title "Edit Account".
2.When you successfully found it, then fill into field inder it your current password.
3.Under that field you can see another field under title "Username" -write there your future username.
4.When you have it done, leave all other fields blank (if you don't want to change anything else) and click on "Save".
5.Now you have successfully changed your username on web and also in game.
Please consider that, you can change your username only once per limited time.