When affiliates register with your shop, they first receive your default commission rate. You can, however, set individual rates for your affiliates. To do so, first click on the "Affiliates" button in the navigation to the left.
Once there, find the affiliate whose rate you want to change and click on their email address. You can use the search field to filter results and the column headers like "Email" and "Name" to sort your affiliates.
Once you've found the right affiliate and navigated to their individual page, you will see a display of their information as well as a field to change their commission rate on the right. Change the number to the desired rate and click "Update Rate." That affiliate will now receive the new commission rate on future conversions.
Please note that your affiliates are notified when changes to their accounts are made.