For a single comp:
You can search in the timeline panel search box for the name of the asset you wish to replace, then select them all when the timeline is filtered. Then you can use Alt+drag to replace a selection of layers in a comp with a single layer from the project panel.
However, for global changes:
You can right-click any footage element in the project panel and choose “Replace with Precomp” to replace all instances of its use with a precomp. Then, by editing the precomp, you affect all uses.
You can Alt+drag one item in the project panel onto another to replace every instance of its use.
You can right-click any item in a project panel and replace the footage it refers to.
Walter Soyka
Designer & Mad Scientist at Keen Live [link]
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
@keenlive | RenderBreak [blog] | Profile [LinkedIn]