From within the course navbar, click on the 'Tools' tab and select 'Classlist' from the dropdown

Find the student you need to edit accommodations for and click the downward arrow next to their name. From the dropdown select Edit Accommodations

From within the Edit Accommodations box you can select the Modify Time Limit box and choose either the Multiplier of original quiz time or Extra time. Once completed click the blue Save button.

Once saved, the student will have an icon next to their name that you can see that shows they have accommodations. Moving forward this specific student will now have those accommodations enabled for every quiz they take.