• Try safe mode if your Mac doesn't finish starting up - Apple Support
You could also see if the system's Disk Utility can be used to check or repair
the hard drive; (using the word 'hard drive' even if the unit may be an SSD)
by use of Recovery and the OS X Utilities found in your Mac.
• OS X: About OS X Recovery - Apple Support
That also be used to do much more including online re-installation of the OS X.
Recovery may be handy if later you were to replace the hard drive, etc.
It may be helpful to access the computer and see about making a backup of
anything you've created or saved into the computer; then you could consider
removing the excess from the computer so it can run correctly.
An externally enclosed hard disk drive can be a place to store excess content
and then once that content is safely archived and accessible, erase or trash
your originals. You may also consider online backups or places to cache your
files; I have duplicate email accounts online and do not download them, for an
example of how to not store items in a computer. Images and music libraries
can be backed-up to external drive devices. And the associated applications
can be pointed to the new archive locations, if done correctly.
Or a new larger capacity internal storage drive may be installed in the computer
and the content of the current one transferred or cloned into the new one. An
authorized Apple service provider could do this job for the cost of labor and the
new drive; be sure everything is backed up to an off-computer archive no matter.
The priority is not to update the OS X, the priority is to attempt to sort out the
over-full hard disk drive and learn how to archive the content you've saved in
a way that could be accessed and used at a later time. And learn what is junk.
To be careful to not get or use gimmicky third-party cleanmymac junk or get
applications from untoward or illegal sharing sites that are known to load them
up with junk or adware, malware or defective software they then may want to
offer to sell you a fix; as those often damage most everything & cure nothing.
Hopefully someone with more time can look at your thread & offer more ideas.
The time here is nearly 2AM locally, so I'm going offline.
PS: is your portable Mac a new MacBook 12-inch model or another one?
In any event...
Good luck & happy computing! 🙂