[2021] How can I change my In-Game Username?

You can't. The name in Eco is the steam name with which you joined the server. If you change your steam name and then freshly join a server you will have a different name.

As I said above, the name cannot be changed once you already joined a server, it must be a new one or the same after they have reset their map. The name you join a server with stays until the server is reset.

Originally posted by Seedy: Or at least an error check to discard bad characters in the name, so you don't end up seeing a dozen people named "Unnamed #2 #3 #4" or variations.

Invalid characters are replaced with spaces, "Unnamed" appears when all characters would be invalid and the name would hence consist only of spaces, which is invalid in itself. We do plan to add the ability to choose a Eco specific username now that we have persistent avatars.