It does depend on the version of Altium you are using, but essentially what you need to do is to select the schematic symbol and then select the parameter section below, Footprint, and choose an existing footprint. If this is not the one you are after you need to select the "pen" icon that opens up another window showing your various things amongst which is also the current library source. If this is not the source of your footprints then click on the ".." on the right side of the current library name that is currently selected. A window with "Available File-based Libraries" (or similar) will pop-up allowing you to choose another library source from your Windows file system path. Choose the correct library that contains your footprints, close etc. then choose the correct footprint for your symbol.