I bought a seat reservation on ICE by mistake for 4.5 euros and I then realized I should buy the ticket so I bought the ticket and realized no reservation was necessary. Is it possible to cancel the seat reservation? If yes, how so?
Seat reservations can be changed once, but usually not cancelled - see the DB website on changing or cancelling tickets and reservations.
Depending on the applicable conditions, seat reservations can be changed free of charge once, regardless of whether you made the reservation online or, for example, at a DB Travel Centre.
The page provides further details on different types of reservations - as you state you paid for yours, it should fall under the category Non-complimentary seat reservations, for which the detailed conditions are as follows (emphasis added):
■ Seat reservations purchased in conjunction with a flexible fare can be changed once free of charge up until the first day of validity for a rebooked journey taking place no more than 31 days later (subject to availability). Seat reservations purchased in conjunction with a saver fare can only be changed on the day of validity of the reservation. Obligatory reservations are excluded from exchange. You can make your new reservation online, via the DB Navigator app or at a DB Travel Centre. Refunds are not available.*
■ Exchanges and refunds are not available after the first day of validity.*
*Exception: If you were not able to use your seat reservation for reasons attributable to Deutsche Bahn, the reservation fee will be refunded. Possible reasons attributable to Deutsche Bahn: Your coach was not available. Your reserved seat was not available or not usable. You were not able to use your reserved seat due to a delay and missed connection.