I have no space. How do I download with not enough space

Man, these 128GB hard drives are so annoying. We had the same issues with Windows 10 over five years ago. There was enough room for the OS and major Apps but so little free space for user data. I am glad that Apple no longer ships Macs with 128GB storage options.

Here's what I recommend. Get yourself a couple large external HDD or SSD drives. Make a full backup of your working system with Time Machine or Carbon Copy Cloner or SuperDuper to one of the external drives. Then use the second external drive to move your data over to free enough space. Things like your ~/Pictures ~/Movies ~/Documents folders. Move your biggest Apps in /Applications such as GarageBand and iMovie and any video games to the external drive.

Then run the macOS upgrade and when it's done, move the files back. You'll have the backup in case anything goes wrong. As far as the amount of storage you should buy that depends on how much you spend. I would recommend SSD for performance reasons but HDD will work fine for a Time Machine drive as you are not moving a lot of data after the initial backup and it's just incremental backups. A Time Machine drive should be at least 2-5 times the capacity of your internal storage. A 1TB external drive for Time Machine would allow you to keep many files over a long period of time before you fill it up. The drive used to move things around and give you some extra elbow room could be the same size or perhaps 480-512GB which is more than double the storage of the internal drive.

The files that you have control over are located in the Documents, Downloads, Pictures, Music and Movies folders.  You can use either of these two free apps, GrandPerspective  or OmniDiscSweeper, to find the largest files on your drive so you can determine if they can be deleted or moved to an external HD for storage.  

Note: you can empty the Downloads folder after the apps and/or updates that were downloaded have been installed or applied.  Many users have found a couple of Gigabytes of files in their Downloads folder which are no longer needed. 

Also, to make it easier to create screenshots and add them to a post so we don't have to turn our monitors on edge read the following:

How to take a screenshot on your Mac 

(⌘⬆︎3) Command-Shift-3         Capture the screen to a file

(⌘⬆︎⌃3) Command-Shift-Control-3 Capture the screen to the Clipboard

(⌘⬆︎4) Command-Shift-4         Capture a selection to a file

(⌘⬆︎⌃4) Command-Shift-Control-4 Capture a selection to the Clipboard

(⌘⬆︎5) Command-Shift-5         Invoke new screen grabber

Paste the following command in the Terminal window followed by the Return or Enter key to change the type of screenshot file from .png to jpeg:

defaults write com.apple.screencapture type jpg && killall SystemUIServer

Exit the installer and free up disk space.

Free up storage space on your Mac - Apple Support

For upgrading a macOS you need about 45GB free