I currently am using Rasbian but I want to switch to OSMC. I don't know who to exit raspbian or uninstall it to get back to the list of operating systems
There is no way to reverse the installation of Raspbian (or any other OS). If you have important data which you would like to save, I would recommend making a backup of your files, and maybe even a copy of your Raspbian image. For more information on that, check out how to back up an SD card.
Once you have retrieved everything you want from your current installation you have two options:
If you are prepared to manually download and flash images, you can wipe your SD card and install either Noobs or any other compatible OS directly.
If not, you can by a new Noobs SD card and start again.
Note: I believe there is a way to install RaspBMC or Openelec from Raspbian. See Dougie Lawson's answer on the raspberrypi.org forum.
If you are using NOOBS you can get back to the installation menu on reboot. (You should state this in your question, but the other statements implied that this is the case.)
Press and release the shift key repeatedly during boot. Only one press is needed if done at the right time. See https://github.com/raspberrypi/noobs/blob/master/README.md#troubleshooting
You can then install a new OS (or a couple if SD Card size permits). NOTE This will totally remove the OS and any configuration you have performed.