You can't increase payments on account towards the following tax year – you can only reduce them. You can choose to pay more on account, but HMRC will only ask for the payment on account calculated in your statement. To reduce payments on account, please follow the below instructions:
Tick box 10 on the tax calculation page and enter the new amount in box 11.
Working out your tax > Tax Due > tick the box 'You can claim to reduce the payments on account. You should do this if your profits or income are not likely to be the same as the amounts shown.....' > Enter the amount in box 'Please enter reduced amount of your first payment' and enter the reason why you are reducing your payments on accounts in the box provided.
PLEASE NOTE: If you reduce the payments on account to be lower than the following year's tax liability, you or your client can be charged interest if there is a balancing payment remaining.
You need to complete the above information on 2016-17 tax return to reduce payments for 2017-18. If you've already submitted your tax return, you can post the SA303 form (claim to reduce payments on account) to HMRC.