To find the quizzes in your course start by clicking on the Course Admin tab in your course shell. 

Scroll down and click on "Quizzes" under the Assessment section

Scroll down and click on the caret next to a quiz you would like to edit dates for, then click on "Edit"

You can set a due date in the middle of the page above the quiz description. Click on "Availability Dates & Conditions" on the right side of the page to enter Start and End dates.

Click on the blue "Save and Close" button when finished.