Hello whited Thank You for posting your question. You might want to check out this "Canvancement" (Canvas Enhancement) created by @James called Adjust All Assignment Dates on One Page. His solution uses a Google Spreadsheet to modify due dates. He provided detailed written instructions and short tutorial videos throughout his documentation so that you can start using this right away.
I hope this helps, Deena. Please let Community members know if you have any questions about this...thanks!