If you change a value (e.g. the “Amount”) of a planned transaction (e.g. in the “Planned” view), you automatically change all future transactions as well. However, if you only want to change e.g. the amount for the next transaction date, you can either increase the amount for the next planned transaction in the “Planned” view now, and then (after that transaction has been debited) change the next planned transaction back to the previous amount.
Alternatively, you can simply wait for the transaction to be debited and then change the amount of the debited transaction in the “Transaction” list to the desired value. If you then select “Save only this” when saving, your next planned transaction will remain at the previous amount.