If you want to do it on the command line, you can do as follows (n.b. you have to do this on the folder containing the external folders):
[[[
D:\Dev\tortoisesvn\ext>svn propedit svn:externals .
Set new value for property 'svn:externals' on '.'
D:\Dev\tortoisesvn\ext>
]]]
Initially, I didn't have an editor configured for my system so I actually got an error message first and I had to set the EDITOR environment variable first, but you can do this in Windows if you want to.
[[[
D:\Dev\tortoisesvn\ext>svn propedit svn:externals .
svn: E205007: None of the environment variables SVN_EDITOR, VISUAL or EDITOR are set, and no 'editor-cmd' run-time configuration option was found
D:\Dev\tortoisesvn\ext>set EDITOR=notepad.exe
]]]
Since this is a TortoiseSVN forum, I would also suggest that you can change it using the context menu in the containing folder.
To make the changes permanent, you have to commit it as with any change.
Hope this helps!
Daniel Sahlberg