To add the new subVI as a node on other diagrams, locate the subVI in the Project Files tab and drag it to the diagram where you want to use it.
All instances of the subVI are automatically updated with most changes you make. However, if you remove or rearrange inputs and outputs on a subVI, wires attached to those inputs and outputs break in VIs that call the subVI. Fix the calling VIs by deleting the broken wires and rewiring the updated inputs and outputs.