Home tab > Symbol group > 
Edit menu > Load
Change a symbol definition in the drawing to a symbol file on the directory.
Use this function when you have inserted a symbol to the drawing and later exploded and modified it and saved it into the drawing but not to the directory.
Do the following:
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In the Symbol file name dialog, select the symbol that you want to replace in the drawing.
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Click Open.
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Enter the name that you want to use to store the symbol on the drawing.
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If the symbol exists in the drawing, select whether to replace it.