This is what I found in Excel's help

Change the source workbook for all links that use the source

Open the workbook that contains the link. 
On the Edit menu, click Links. 
In the Source box, click the name of the link whose source you would like to change. 
Click Change Source. 
In the Change Source dialog box, click the source workbook you want to refer to. 

I don't want to have to give instructions to my user.

I want to be able to select the source from a "validation list" and have it automatically change the source of all links.

Alternatively, the user would select the source from a "validation list" and then click a button to change all the sources.

Alternatively, when the workbook opens, it has the user point at the source she wants to use.

I can't get a macro to allow the user to select a workbook.

TIA