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Navigate to the Edit Quiz screen of the applicable quiz
Click on Manage Special Access in the Availability Dates & Conditions pane on the right:
Click on Add Users to Special Access
Check the Override time limit box and provide a new time limit (or no time limit). Note that you can enter a number in any box and the others will automatically calculate (eg. changing 1 to 2 for Multiplier automatically filled the other two boxes as seen below
Scroll down and check the boxes next to each student this accommodation should apply to
Click the blue Save button at the bottom of the screen
Click Save and Close