Changing your username will log you out from all other computers/devices. Choose a strong password and don't reuse it on any other accounts.
Use atleast 8 characters / 1 upper case letter / 1 lower case letter / 1 special character / 1 number. Don't use a password from another site, or something too obvious like (33333333, 12345678, abcdefg, pet name. I like hot dogs with mayonesse. I like sausage
