You can remount a linux system on the fly like this:-
umount tmpfs
The above command will ask the OS to unmount the filesystem identified by tmpfs in the fstab file.
This may return an error if you are actively using it – i.e. you have an ssh session open and are in the filesystem or open files exist. – Exit any of these and try again 🙂
mount tmpfs
The above command will attempt to re-mount the drive.
When would you use this?  Normally if you have done some dynamic re-sizing of a partition  – for example the tmpfs partition – see post:
http://www.ihelm.org.uk/2010/04/resizing-ram-disk-in-linux-devshm/
for more info on how to do that.