PM one of the mods to do it for you. I dont think you can do it yourself once registered. If you look on the home page on the left hand side you will see a list of who is on line and the names in bold are the moderators. If you PM (private message) one of them they may be able to do it for you. Click on their name and you go to their profile and then click on private message.
Welcome to YD. Unfortunately the Mods are too lowly to be able to change usernames .... that has to be done by an admin. Please send a PM to andy2tanx who I am sure will oblige. I think that Linda has already been taken so you might want to suggest a couple of options for him.