Highly depends on which route you are travelling because almost every railway company has its own rules.

Could yo mention the exact train, travel date and route?

In general, a seat change or a exchange of your reservation is usually possible for Eurostar and TGV trains.

My ticket &my wife ticket are same train

But my seat is too far from my wife

So I'd like to change one seat of two so that

Sit together or move to same  coach at least

October 26th,Aix-en-provence TGV(14:18)  →

                        Paris gare de lyon(17:22)

My Booking number:EAJKBN

E-ticket number:259742752(coach 6  seat 46)

My wife's booking number:KBN3PX

E-ticket number:884969054(coach 8 seat 113)

Please  adjust them

Exact train  : TGV6120

Seat reservations for TGV are fully refundable until 30 minutes after departure. So I see two options:

1. Cancel your reservations through the place you bought them (in your case Eurail). I don’t have experience on that topic so maybe contact the Eurail customer support if you aren’t able to cancel the reservations otherwise. Buy new reservations afterwards through www.raileurope.com (Desktop version, click “add railpass”). No booking fees and you are able to make two reservations in one go. I don’t know if you are even able to see the seat numbers before paying. It’s for sure the better place to make Eurail reservations on this site than through the Eurail reservation portal.
2. You can also call SNCF (French railways) if you want. They might also be able to change your seats.