Put your composition in a composition of the frame rate you need your end product to be.
If you have rendered this project and you like the result you can do two things to convert the project to broadcast frame rates. The first is to do what is done to all films that are digitized for broadcast. Add 3:2 pulldown.
The second is to do what Szalam suggested, simply drop your original composition in a comp with a different frame rate. This may or may not work well. It depends on the footage and the motion in your original comp. Personally, if I had designed and approved a project at 23.976 I would never just drop it in a 29.97 comp and let the frames and the motion fall where they may. It will work, but you may end up with some funky frames here and there.
To add 3:2 pulldown Add your original comp to the render cue then open up the Render Settings and set interlacing to upper field first and add 3:2 pulldown. You will notice the frame rate on the right side of the window will change to 29.976.
Then render a production codec copy of your footage, drop that footage in the AME and render your 1080i H.264 mp4 file for the broadcaster. This will give you the best representation of your 23.976 fps project in a broadcast situation.
BTW... ALL broadcast in NTSC land is still interlaced and 29.976 fps. You can deliver progressive footage to them, but when it is encoded for broadcast it will be interlaced. There will be no difference between field 1 and field 2, but it will be interlaced.
Also, don't forget to check audio specifications and make sure that your audio is no longer than :28 and the spot is :30 seconds long when you have Drop Frame timecode selected. These little details will cause a spot to be rejected.
I made a commercial for my work, in AE @ 23.976 fps (1920x1080). Now my work wants to air the commercial over TV.
The specs that the TV station wants are H.264, frame rate @ 1080i - 29.97 or 720P - 59.94, size 1920x1080 or 1280x720.
How can I convert the project from 23.976 to either 29.97 or 59.94? Will the audio and video sync if I change the fps in the render setting? Please advice.
There are other specifications you need to conform to such as color space and white levels and audio energy range. Not all of these are automatically handled by After Effects. Open up (the rarely used) Color Finesse app and learn how to use those scopes after you have obtained the proper specs.
There are no digital broadcasters in the States using full 1080, everything that goes out over the air is 720. Full HD material is scaled down to 720 as it enters or leaves the control room. Waste of time providing them with full HD files.