I guess you mean for this account: https://www.opensubtitles.org/en/profile/iduser-1266736
I can imagine you don't want to loose the uploads and the Trusted status.
The username cannot 'just' be changed as a standard user option. If at all possible, this needs to be done manually and could only be done by the superadmin oss in the database.
Another option is to just make a new account, submit 64 reports for a change of uploaders name (yes, a bit time consuming...) and ask admin MaSousa to make you Trusted again.
Or just live with that username - what's wrong with it...