It has a big nut on the bottom of it. You have to remove this nut and loosen the clamping nut at the P-trap and then it should lift out. It has cross shaped things in side of it - a Channel-Lock's handle works good to keep it from turning while you unscrew the nut. It should be siliconed in. You never wan't to use plumbers putty on a plastic tub - use fresh silicone when installing the new drain.
I strongly recommend using either a chrome/brass or plastic drain. The cheap chrome ones will not last nearly as long - they rust out quickly.