Hi Sarah, thanks for getting back to us here.

We don't store the Government Gateway information in QuickBooks. If you think the sign in credentials need correcting, you should disconnect and reconnect MTD. 

To disconnect MTD, go to the Gear icon and select Account & settings, then Advanced and Making tax digital. From here, click Learn how to disconnect and follow the on-screen flow to confirm the disconnect. We'll be here if you need any further support!