Just did this a few weeks ago...but I did it when I replaced my stock cassette player for an Kenwood player
1.take off center console
2.unplug antenna connection from the back of your unit
3.mine had an extension off some sort with clips I didn't need it so I unplugged it
4.pull antenna out (the antenna wire should run from the back of the unit and kinda behind the kicker panel and all the way to the left behind the fuse box and hood latch opener)
5. I would suggest before pulling it all out to tie a long piece of string to the end of your old antenna connection(the part that was connected to the back of ur unit) so it could serve you as a guide for your new one
6. If you skipped step 5 like I did

your going to need something to guide it back in from the roof of your car all the way behind the fuse box...I used a coat hanger and took me several tries because you can't really see the hole can only feel it
7.after that you pretty much just do the reverse in my case I ran the new antenna cable to my new head unit before installing
And that's it

hope that was clear enough