I know of no website that allows you to change your username (which, apparently, is what you're trying to do) while retaining all of your old data (such as your ratings on auction sites or questions and answers on sites like this one). In every case you need to register a completely NEW account, which usually means using a DIFFERENT email address.
(The Editor did, at one time, have a brief 'amnesty' whereby members could ask for their username to be changed but there is no automatic way to do it).
So if you want to become 'Liesch' on here, you'll need to get a DIFFERENT email address (e.g. at
http://www.gmx.co.uk ) and register again as a new member, using your preferred choice of name.
Your old posts (using your old name) will, of course, still be here. If you've got a reason why you'd rather they weren't, simply post a message - still using your old username - to that effect and ask to be banned; then your old posts will all disappear.