how do i get more storage on my mac so i can update it

You can not increase storage by increasing the size of the HD. What you can do is to either delete files to create needed space or you can install an external disk. You can then copy files onto the external disk (such as movie and photo files) which can then be deleted from the HD. These days external drives are cost effective, compact and easy to install with vendor provided USB cables. Most do not require an external power source. Problem solved. Note - if your disk becomes too full you will not be able to download and install OS updates or upgrades and it can even stop functioning.