Not available at this moment.
Warning: Ubisoft aren't currently offering an Apex Edition upgrade package for Far Cry Primal Standard Edition users on Steam.
Far Cry Primal Apex Edition currently costs $5 more than the Standard Edition and includes three extra missions, a new weapon, and four enhancement packs.
Ubisoft didn't mention if they'll offer an upgrade package in the near future.
If you had second thoughts about not purchasing the Apex Edition, Steam are offering refunds for Far Cry Primal Standard Edition users as long as they: 1. Submit a request within two weeks of purchase; and 2. Played that title for less than two hours.