I need help to change my apple ID

I have an apple ID and a personal e-mail address, somehow i have used my apple ID (@apple.com) to sign into iCloud etc and now i want to change it to my personal e-mail so that it is easier to remember. However when i try to edit it it says my personal e-mail is already assigned to someone else, i don't know how this has happened or how to change it as once i get to that point there is no option for further help. Do i need to speak to someone at an apple store or can it be sorted online?!

Any help much appreciated!