Pay pal credit buy now pay later, paying money off one specific item

Description

I currently have three purchases on paypal credit, One was on twelve monthly intsallments when I have been paying a direct debit each month I worked in what should have been the twelve monthly installments but paypal has been sharing the money equally amongst all three, so now one of the buy now pay later is due in december it was over four months and the twelve month payment plan will require less.
How can you actually pay off one specific item so this does not happen ?