Nobody can tell you anything without proper system info or other technical details like version of AE, what footage, where it's stored and so on.
Has this ever worked? What kind of footage? There are two things that you may be trying to do and I don't know which one. I'm guessing that you have some footage in the comp window named something like myMovie.mov and you want to replace it with yourMovie.mov. The normal procedure is to select the myMovie.mov in the Project Panel and either right click and choose Replace Footage or go to the file menu and select File>Replace footage. If the selection is not available then you have either not selected a supported footage type or you are are on an internal object like a shape layer or a composition. Please let us know what your are trying to do.
The other thing you may be trying to do is to select a layer in the timeline and replace it with some other asset from the Project Panel. To make that work you select the layer in the timeline first, then you select the asset in the Project panel and hold down the Alt/Option key and drag the replacement to the timeline. This works with anything in the Project Panel.