How can an adjustment to the amount payable on the vat return by mistake. how do I reverse this entry to make the payment amount correct?

Let me help you make a VAT adjustment in QucikBooks Online (QBO), geoff50k. 

For errors made in previous VAT returns, we'll need to inform HMRC and complete the form VAT652. We can also include the adjustment for the errors on your next VAT return if your VAT errors don't exceed £10,000 and 1% of the box 6 figure required on the VAT return for the period of discovery (subject to an upper limit of £50,000). 

To make adjustments to the amount payable on the VAT return, I'd suggest consulting your accountant on what account you'll need to apply on the boxes. Just ask them if you need to apply an income account or an expense account for box, 1, 2, and 4. adjustments. Here's an article you can refer to for more details about making VAT adjustments in QBO. 

Just in case you'll want to adjust your VAT due (if you use Automated VAT), see the Create or delete a VAT adjustment for the steps and details. If you need assistance with the provided steps, don't hesitate to contact our Customer Support Team. Here's how to reach them: 

1. Click the Help menu in the upper-right-hand corner.
2. Type in "Talk to a human", then press Enter.
3. Look for I still need a human and click on it.
4. Click Get help from a human or Contact Support Team.
5. Select between Send a message, Schedule an appointment, or Get a callback.

To ensure that you'll be assisted immediately, I recommend checking our support hours before performing the steps. Feel free to visit our Taxes page for more insight about managing your taxes on your account. 

If you need to take care of other things in QuickBooks, I'm with you every step of the way. Just leave a comment below and I'll get back to you. Have a great weekend.