Well an update won't change it turning on and off on it's own, nor would it help with slowness. Also, if there is nothing listed within the tablets updates (in settings) then there is no update for that device.
What you can do to speed it is up is first check it for viruses, malware, bad apps, etc. If any are found, remove them and then restart the device (turn it off and back on).
Should that not be enough, you may want to consider backing up all your personal stuff from the tablet to a google account or a computer and then "Hard Reset" the tablet. This would wipe everything on it and allow you to start fresh. So unless there is a hardware issue, it would run smoother after that.
NOTE: Doing this will clear 'all' the data on your...