To update commission on a marketplace contract you already have with a seller. You have to update the commission on the Contract terms that are connected to the marketplace contract.
To do that start by selecting "contracts" on the top bar, then select "contract terms" from the drop down list:
Select the contract term you need to update from the list of available contract terms:
When you open the contract terms you come to the page where you add the commission for your products. To change e.g. 20% to 15% just add 15% to the field. Here you can find more detailed information on how to edit the commission and what options you have to set it.
Next, you need to save the changes you made on your commissions.
The last step is to update the contract on the marketplace with your seller. You can do that here in the Contract Terms by selecting the "Sellers" tab. There you can send a new proposal to all the sellers that are connected to this particular contract terms:
To update the contracts, select the checkbox in the front of the line to select the seller you want to update the contract for. Write a comment in the comment field and then select "propose amendment". Now there is a amendment proposal waiting with the seller, that he has to accept for the changes to take place.