To change your username or password:
Go to My eBay > My Account > Personal Information
Select Edit next to the one you'd like to change, and follow the instructions.
You can only change your username every 30 days.
This will change your eBay ID but nothing else so your items, messages, feedback etc will be associated with your new name. There will be a little Icon beside your ID for about 30 days to show other users that the name has changed.