Changing the cell values by macro to a cell in another column

I need a macro that sets the value of selected cell to the cell in column "B" which has the same row number with selected cell.
Example :

When I run the macro when the cell A3 is selected it should make the A3 equal to B3. (selected cell still should be A3)
Or when T39 is selected macro should make the value of the cell as B39. (And after the process the selected cell should be still T39.

Thanks in advance.