To manage your payments, you will need to select a single trading partner in your search criteria on the payment allocation page, and then click on the Unallocated receipts button to display a list of your currently unallocated payments.
Next, select an unallocated payment from the list and, as long as you have edit access to the selected trading partner, you will be able to click on the following options in the bottom right-hand corner of the page, depending on the desired task: