Try each of the following steps until the issue is resolved:
  1. In Tickets, Configure Daily Sales Page, under Default Daily Sales buttons, ensure the Mid-term Upgrades button has Active as Yes.  If not, click Set button order and move the Mid-term Upgrades button from the Inactive buttons list to the Active Buttons list using the center blue arrows.
  2. In Daily Sales, Patron field, select the Patron name.
  3. If this Patron is eligible for a mid-term upgrade, the mid-term upgrade button will now appear.
  4. If the button is still missing, go to Memberships, select the affected Membership program name.
  5. Ensure this type of membership allows mid-term upgrades.  Go to the Terms and Levels tab.  Is there more than one level defined that someone could upgrade to?  Go to the General tab and ensure this is a dues based membership program. 
  6. If the membership is dues based and has multiple levels, on the Terms and levels tab, ensure that the current level has a higher level listed below it in the list.  The list of levels should be ordered lowest to highest from top to bottom. (lowest level being the first level in the list).
  7. If the levels are incorrectly ordered, highlight the row and click Promote or Demote to move it in the list. 

As an alternative solution, go to Constituents > Search and search for this constituent.  Go to the Memberships tab and see if you can click Upgrade here to upgrade their membership in the back office instead of Daily Sales.