You can bulk update questions to set the mandatory state on each. Just follow these steps:

1. Go to List Questions on the left menu bar when viewing your survey
   
2. Scroll to the bottom of the page, and change the displaying rows per page to the max (100). Assuming you don't have more than 100 questions, you only do this once.
   
3. Scroll to the top, and click the checkbox beside the text "Question ID" to select all displayed questions
   
   
4. Scroll to the bottom, and click the "Selected question(s)" button.
   
   
5. Click "Set Mandatory state"
   
   
6. Set toggle to "Off" and click Apply
   
   
7. You're done, unless your survey has more than 100 questions, in which case you need to scroll to bottom of page, select page #2 (etc) and repeat steps 3 to 7.