So I recently purchased a 03 G35 coupe Auto the guy told me the transmission fluid hasn't been changed in a while the color of the transmission fluid is a light brown so it definitely needs change. My question is with it being 160,000 miles how should I change it should I flush it or just change it and what kind of transmission fluid should I use

 

It's a drain/fill, very easy. Look up a video or a DIY here on the site.

There's a drain plug in the transmission pan, there is no serviceable filter. I think it takes 4 quarts (3.8) and you should ONLY use Nissan Matic-J or Matic-S. Not aftermarket brands.

This will exchange 4 of the 13 quarts in the system, since it hasn't been done in a while just plan on doing a few of drain/fills over the next year (easiest to just do them at the same time as an oil change) to get the old out and the new in. Service interval was 30k miles so 160k / 30k = 5 drain/fills to get you back on schedule.