If you want to change your existing login/user name it can be done.
No you can not alter your profile to change it, only Admins can change it.
The new name must be not already used and must conform to our Terms and Conditions (See the link at bottom of page). You can see if athe name you want to use is already registered here. Simply use the search feature (top right of page) and under the author area, type in the name, and if it says no such user, then it is available.
No I will not change names with frequency, meaning do not be asking to change it every few months.
I hope this helps to answer this topics' questions that you may have had.