Website - MODS HELP!

I have logged in to the MS Website through a temporary password and want to change the old one (as i have forgot the original one i set up) but it won’t let me saying that the username is already taken… i know it has, it’s taken by me!!! Can you help me please? Not sure what was wrong with the old log in details and now the passwords have to be upper and lower case with numbers and punctuation - that’s handy for people with problems with short term memory issues!!! Thanks for your help Allyson x