I boought My Macbook pro 13 (2 )years ago when i opened the cover there is discoloration in lower body. Can I purchase a replacemet lower body?

Replacement bottoms are available online from several sources.

But if you keep using that same cover, you'll do the same thing to the new bottom. Apparently the bottom vents and solid middle are are wearing the finish away.

The picture you posted is not discoloration -- it is wear.

Presuming you are using a FLAT work surface, your battery has expanded enough that the small black "feet" are no longer lifting the center of the bottom off your work surface. The center has protruded below the "feet" and the center is scrubbing on the work surface and is causing the center to appear shiny.

You will need a new battery sometime. Maybe soon.

If the battery swells more, and causes the trackpad to malfunction, it will definitely be time to replace the battery.