You need to contact the office that you bought it from. You probably have to pay the difference in price. I hope it is still possible-I also had a prepaid 2 monthpass for 2020-when covid struck, but the replacement I had to use until end of this year.

If you bought it via the Interrail website, then please contact customer service.

This applies to me as well, and I contacted them last year about it. I can confirm the following for the benefit of anyone who comes to ask this question later.

When - due to Covid hampering/preventing travel - the validity of a pass you bought with the intention of using while still under 28 is extended beyond your birthday, and if that is when you now plan to use it; no difference in price needs to be paid for the functional exchange of the youth pass you bought for an adult one (which is an excellent bit of customer service). You have to wait until you’ve actually had your 28th birthday to conduct the exchange though.

Hi Dillon, a regular exchanged will be applied to your case- our deadline for free exchanges in exceptional cases was the 1st of September. This means that you can buy a new Pass and we'll refund the old one for 100% minus 15 euros (exchange fee).

Hi Ed, I’m in exactly the same situation. Do you know how I can exchange it to an adult pass?