How do I convert 120FPS video from the iPhone 5s to 60FPS?
Update2: Official Apple Support iMovie (2013): Slow down and speed up clips:

Make a clip play more slowly
In the timeline, select the clip you want to slow down.

Choose Modify &gt; Slow Motion, and choose a speed percentage from the submenu.


Update: Maybe i misunderstood the question, you just want to make it half the FPS without changing the play speed, right? iMovie 11 for Mac is supported to export 60 FPS videos:

So, here is what you have to do if you want to export a 60p movie made with your 60p footage :
- create a NTSC (29.97 fps) project in iMovie
- choose "export using quicktime" in the "Sharing" menu
- choose the correct codec settings (I choose a Quicktime movie with the H264 and AAC codecs, max quality, and recompress the movies with Handbrake afterwards)
- be sure to check the correct framerate (59.94 fps for my 2 cameras, I guess it should be pretty standard - you can always check it in the information window in Quicktime Player for a raw file from your camera)
- export and enjoy.


Source

You can slow it down using iMovie on iOS:



Or you can use different applications for record, like SloPro - 1000fps Slow Motion Video.