I don't think that SQL is the way forward in this case; you will need to use VBA or a Macro. Unfortunately I don't know VBA for Access so I can't give you a definitive answer.
Create a Message Box containing a input field to contain the new value or multiplier and two radio buttons (update and multiply). Show the message and store the new value/multiplier
Find the VBA command to get the selected cells, in Excel this is Application.Range.
Iterate through the selected cells and set each of the cells to either then new value or the current value * the multiplier.
I can't find an example of how to get the selected cells in Access but I hope that this helps