To change a Partner's commission value referring to a reservation, there are two possibilities:

During the manual creation of a reservation

In the process of creation a Stays.net reservation, in the [Conditions] step you will be able to define the partner through the [Reservation Type > +Relations] field and, if necessary, change the commission value.
You will be able to define the fixed value (e.g: R$ 100.00) or a percentage on the daily rates.

Reservations already created previously

For reservations already created on your Central, the alternative is to access the reservation page and, in the area with Partner's information, click on the little pen icon next to the word [Commission].
You will be able to define the fixed value (e.g: R$ 100.00) or a percentage on the daily rates.

For reservations imported from Sales Channel where it already informs us the value of the commission, Stays.net will not allow making this action so that information is not misaligned between platforms.