believe it or not, if you know how to let users register, you are pretty much there, code wise, in allowing them to edit it.
Just have some sort of trigger that a user can click (ie a link to a switch statement such as user.php?do=edit&user=blah)
then in the switch statement query the user's info then include a form that displays text fields and in the value for each field put the appropriate column name (ie value='memberId', value='password')
hope this helps...
EDIT: also, don't forget to session register whatever variables you need so when a user successfully logs in and clicks edit, he/she will be editing the appropriate fields and not their friendly neighbors!