You will need to communicate the desired change of price with the traveller, either through the comments or private chat function. If a price top-up or partial refund is agreed on for the request, the traveller will have to re-offer at the newly agreed price. You will then be able to review the changes submitted by the traveller before accepting or declining the re-offer.
For price top-ups, you'll be required to make the payment via the same mode of payment as your first deposit. Partial refunds will be processed immediately upon accepting the re-offer. It may take up to 3-10 days for the refund to be reflected in your statements, depending on the bank and mode of payment used.