Use Properties|Externals to define or change externals. An external (officially also referred to as externals definition) is a mapping of a Local Path to an URL (and possibly a particular Revision) of a versioned resource.
In general, externals are specified by complete URLs, but there are also shorter representations which can be more flexible. The URL input field allows to switch between the available representations for a given URL. For a detailed description of externals and valid URL formats, refer to http://svnbook.red-bean.com/nightly/en/svn.advanced.externals.html.
It is safer to always set a Revision to externals. In this way you can always be sure about which version you are actually working with. When you decide to use a more recent revision of the external, you can evaluate it beforehand and, if you are satisfied, increase the Revision number of the external definition.
Externals may refer to directories as well as to files.
In case of files, the referred URL must be part of the same repository to which its
local parent directory belongs.
(The local parent the directory is the directory to which the