If your wireless mouse is laggy, as with all wireless accessories, change the batteries first.
After you change the batteries, you may have to resync your mouse. First, just try to put some new batteries in and use it. If it doesn't work, flip the mouse over. If you see a green or red 'sync' button, find the receiver for that device, which you would normally find on your desk, behind a monitor or tied up under the desk. Press both the sync button on your mouse and the receiver at the same time, and the mouse should be good to use again.
If you replace the battery for the mouse and it is still malfunctioning, it may be due for replacement.