Go to the Vendors section and click the Vendor whose Master Contract you'd like to extend. Then, press the pencil icon next to Extend End Date.
Pick a date and save your change.
Finally, you can also send your Vendor a new invite with new Master Contract dates. When you send your Vendor a new invite, the resulting documents will appear on that same Vendor's card, provided the Vendor accepts that invite with the same account. If the Vendor creates a brand new business to accept the invite, then the new documents will be shown on the new Vendor's card and not on the original Vendor's card.