You can change the target of an xref from local to external choosing the new target from existing documents.
Do the following:
To call up the
Change Local Xref Dialog, choose or click the
Insert Local Xref button
on the toolbar.
The Change Local Xref Dialog will appear.
Figure 1. Change Local Xref Dialog.

To change the target of the xref from local to external, click the External Xref tab.
In the Change External Xref Dialog, specify the target file one of these ways:
From the Files list, select a file.

To filter the files by type, select a type from the File type drop-down list.
In the Path field, type the path to the target file or URL .

URLs must start with " http://", e.g. http://www.syntext.com.
It is also possible to type the reference to anything within a topic, using the following format: filename.xml#topicid/elementid.
Press Enter to apply the typed path.

From the Targets list, select the target.

You can see the list of available targets in the current document.
To filter the targets by type, select a type from the Show types drop-down list.

dita
html
ditamap
local
peer
external
You can also check the Close on Change checkbox, and then click the Change button.