Not enough storage for El Capitan; no way to free up space I wanted to upgrade my Macbook 2008 from Yosemite to El Capitan.  Unfortunately, when I downloaded it and restarted my computer, on the "select the disk on which to install" page the only location, Macintosh HD, didn't have enough space (by about 2 GB).  It doesn't seem like there's a way to free up space from this startup menu, though.  I can open Disk Utility from this menu, and it seems logical to put some method of freeing up disk space from there, but if there is, I can't find it.

Is there a way to get out of this startup menu so I can free up space, or a way to free up space directly from the menu?  This is pretty frustrating because it seems like a major design flaw if it tells you you don't have enough storage for a certain thing, but it doesn't let you free up the storage to download the thing.

There's already a question exactly like mine: Stuck in update boot (not enough disk space)
But no one has answered it, so I think it's okay to ask it again.

EDIT: I can also access Terminal which I could potentially also use

EDIT AGAIN: Nevermind, it looks like Macintosh HD is read-only while I'm in this boot menu.  At least terminal says "read-only file system" when I try to use rmdir on something.
Boot to Recovery Mode (hold cmdR while booting). In the menubar open Utilities -> Terminal.

Now change your working directory and go to /Volumes:

cd /Volumes


List all volumes:

ls -l


Move to your main volume

cd name_of_main_volume


Now you can move forward to a directory to remove files and folders with cd folder_name.

Appropriate paths to remove files or folders without touching your own data are:

/Volumes/name_of_main_volume/cores
/Volumes/name_of_main_volume/var/vm


Then remove files with rm file_name or rm *.* after making sure you are in the proper directory with pwd and checking all files with ls -l.

The folder cores may be empty, vm usually contains a sleep image and/or swap files which are all disposable.

If this doesn't suffice go to your applications or your user folder and remove files/folders.

To remove a folder use rm -R folder_name. In this case application files (e.g. Maps.app) are also folders!