To Modify a Symbol in the Library (AutoCAD Mechanical Toolset)

In the drawing area, double-click a symbol of the type of symbol library you want to edit. For example, to modify a symbol in the welding symbol library, double-click a welding symbol.
Click Add.
Note: Step 2 is performed to use the symbol as a temporary symbol library item. You delete it in step 10.
Enter TEMP as the name of the symbol.
In the library, click the edge symbol to edit.
From the drop-down menu on the right, select Load. The selected configuration replaces the information in the dialog box.
Modify the symbol as required.
From the drop-down menu on the right, select Update. The symbol library is updated with the information in the dialog box.
In the library click TEMP.
From the drop-down menu on the right, select Load. The selected configuration replaces the information in the dialog box.
From the drop-down menu on the right, select Delete. The temporary symbol is deleted.
Click OK.