I want to upgrade entertainment system on my 2011 Maxima, right now I have the very basic one without navigation.
Is there a way to replace it with used navigation/premium system? I see them sold on ebay.

And the same question goes for the seats - I want leather seats, I see them sold online but not sure they are compatible with my trim.

I think I have S level trim with no additional option.

 

Your best bet is to just go aftermarket with the audio/nav setup. From memory, the guys that have tried were not successful. It's not plug & play.

 

I went with a pioneer AVH4000 Nex double din head unit and I love it. I has iphone integration so you can use google maps for navigation, take calls and even do voice texting with it.

 

How about the seats? I have dark seats, but I wanted to change to a light leather.
I looked online and it looks pretty straightforward to change, the only potential problem being the sides of the seats. I am not sure I can change them or how hard it is to change.

 

That's definitely something you want to come out looking nice. I would go to a few local upholstery shops & see what they charge....if not, maybe to get some pointers.