If a Receipt is locked and the fields are greyed out, you can't make any changes to those fields unless you unlock the transaction first. 

Firstly, open the Receipt in question and look for the "this transaction is locked" message on the bottom left. Click this message, and you will be advised why this transaction is locked. 

You may need to unallocate it, remove it from a bank rec, or unlock a period. If it's on a VAT return and you can't delete that return now, then you may have to consider posting corrections such as credit notes or negative payments/receipts, depending on the situation.