I want to change both username and password

[DoninGA]

Sign onto the online account with your current user ID.  After signing onto the online account, on the left side of the screen click on Intuit Account.  Edit your information in the Profile section and in the Security section.

See this TurboTax support FAQ - https://ttlc.intuit.com/questions/1900724-how-do-i-update-my-turbotax-account-information