How I can change my username from marrcel0138 into marcel0138?
a) directlink: https://www.openstreetmap.org/user/marrcel0138/account
b) navigation bar top right: click onto username → settings
If I change my username do my past edits carry the new name and do I lose my reputation points at osm help forum.
I’ve never tried it, so I’m guessing, but I suspect that your login at the help site won’t be affected by a name change on the main site.
If that’s true, you have the option of creating a new username on the help site, answering a question, and you can then “award” all the points you want from the old to the new user - or you can just continue to use the old name on the help site.
The software that the help site uses is OSQA. There’s a Q&A site for it at http://meta.osqa.net/questions/ , but it seems not to be actively maintained.