How can I change my seat on a train reservation?

Easiest-but without any garantee on success-is to ask if one of the 2 pax beside friend/you is willing to move/swap. But if that person is also part of a large noisy group (like ITalians so often can be as being on holiday) success will be nil. And-again-not all seats will be taken all the time-maybe the seat beside you/friend remains empty-or in some railways can simply check on the small screens.

Travelling fro Exeter to Paris

What do you want to know ? With a Eurail pass :

- Exeter - London Paddington/Waterloo free (optional) seat reservation

- Paddington/Waterloo - St. Pancras : transfer by Underground (not included)

- London St. Pancras - Paris 30€ seat reservation. Limited quota so book well in advance

You missed the gist of my question. How do change my current reservation to 1st class?

Assuming you have a first class pass and currently have a second class reservation I would think the only option would be to cancel your existing reservation ( Eurostar would probably be a loss). Then buy your new reservations in one transaction The only company I know will offer a first class upgrade price for second class pass holders is SNCF TGVs. This is some element of reservation plus excess for change of class.

If you currently have Eurostar reservations then you can change them but there is a management fee - This can be done online on the official site. I don’t know if you can change to first class though (Standard Premier on Eurostar).