Riot Games' refund system in League of Legends is normally limited to three refunded items but every once in a while they add a new refund token for the players. There is one currently hanging in the air but you have to claim it yourself.
League of Legends can have champions in a really strong spot one moment and then kick them in the shins just days later. In case you picked up a skin for one such champion, it's possible that you want a refund but you're out of the three that you get by default.
Riot Games usually add one refund token directly to your account from time to time but not long after the latest one they gave us, the developers added another refund token that can be manually claimed by the players.
This revelation was initially taken with some scepticism on Reddit since the link to claiming the refund token looked unofficial and many suspected a phishing scam. However, the Rioter that made the claim possible spoke in the same thread and confirmed there is no foul play at work.
In order to get the free refund token, you will need to log in on the free token link and claim it. The next time you check your tokens in League of Legends client, the numbers should go up by one and you're set to return that sweet RP that you spent on something you didn't like eventually.
Riot Games also explained that they intend to make the refund token claims automated in the future but they had to settle for manual ones on the support page linked above for the time being.
Considering that League of Legends' client has been a mess for years, it is not exactly surprising that the team went with the manual token distribution. Some spaghetti code could make it stop working again.