Thanks for getting back here, @Chipndale.

Since QuickBooks Online (QBO) currently no option to file GST F7, as a workaround, you can record the amendment by unfiling the GST F5 and file again containing the correct info. 

For more info on filing GST in QBO, check out this link: File and pay Tax/GST.

Also, here are some of the most common questions asked about GST in QBO: FAQ for GST in QuickBooks Online. 

Don't hesitate to post again here if you need further assistance recording your GST F7 in QBO. I'm always here to help. Take care.