General synonyms:
* Replenish
* Top up
* Reload
* Restock
* Renew
* Supplement
* Reapply
More specific synonyms:
* Recharge (for batteries)
* Reimburse (for financial accounts)
* Replenish (for supplies)
* Retire (for a prescription)
* Replace (for an object)
The best synonym for "refill" will depend on the specific situation. For example, if you are talking about refilling a prescription, "retire" would be the most appropriate synonym. If you are talking about refilling a coffee cup, "top up" would be a better choice.