You can update the commission you have with your resellers in a few easy steps.
Select "Contracts" > "Contract terms".
Select the contract term want to make changes to.
Change your commission in the appropriate fields under "Experiences" and save your changes.
Here you can find more detailed information on how to edit the commission and what options you have to set it.
All that's left to do is to update the contract you have with your sellers on Marketplace. You can do that via your contract terms by selecting the "Sellers" tab.
Select the sellers whose contracts you want to update by ticking the box in front of the seller name. Write a comment, explaining the changes you've made to the contract term and select "Propose amendment". Now all you've got to do is to wait for the seller to accept your new contract terms.