Hello Leisure King Ltd
To change the gov gateway details for the VAT return you would need to disconnect the making tax digital connection in the QuickBooks and then reconnect it .
To do this go to the cog in the top right of QuickBooks>account and settings>advanced>making tax digital>click into that section and select learn how to disconnect>follow the steps and it will take you to HMRC website and you can then enter the right details and authorise it again
Any issues let us know by replying to this post
Emma