Not enough space on the selected volume to upgrade OS

That means that you will have to clear space on the disk so the file can be downloaded. I suggest that you install an external disk. Then you can transfer files, such as photos to the external disk and that will allow you to delete them from your HD to clear up the needed storage space. These days external disks are cost effective, compact, easy to install with vendor provided USB cables and most do not require external power. You may have to reformat it though. There is a large selection of disks to choose from.