Select All Instances (and Replace Them Too)

You can create multiple instances of a symbol by placing copies (Edit>Copy; Edit>Paste), or using the Selection tool (V) to select an instance and pressing-and-holding the Option key (PC: Alt key) while dragging to make a copy. But to select all the instances of a symbol in the document, make sure you have nothing selected in your document and click on the symbol in the Symbols palette. Then use the palette's flyout menu to choose Select All Instances. If you want to replace all the selected instances, click on the new symbol in the Symbols palette and press the Replace Symbol icon (the second icon from the left) at the bottom of the palette.