I want to change my username how can I
You can't, an administrator can, but it was decided my the moderators and administrators previously that we would only allow username changes for memebrs with an established record of contribution, and as a general rule 200+ posts.
You can of course create a new account with your desired username, with only 6 posts including the one here in almost two years, is assume that it wouldn't casue you any problem to start again.