Drain and fill and just stay up to date on maintenance and drive conservative as in don’t step on it all the time. The transmissions on the earlier models are very weak the new xtronix ones are probably better but time will tell.

Definitely drain and fill it should take roughly 5 quarts and use Nissan ns-2 fluid 

Has it ever been serviced before?  If not, I would do it now (drain and fill, with filter change if applicable).  For those transmissions, since they are a weak point, I would stay on top of the transmission fluid changes (and go no more than every 30,000 miles).  Also, don’t drive the vehicle hard and it’s preferable to stick to highway miles as much as you can to extend the life of those transmissions.