Generally there is a button, small and recessed, underneath your keyboard and mouse and a similar button on the receiver that actually plugs into your PC.
By pushing this button and or pushing the button on the receiver a random frequency is picked.
The manual that came with your wireless keyboard and mouse should tell you exactly how to reset it or how you power cycle and push a button so that the keyboard, mouse and receiver "sync up" on a frequency. Do this a couple of times on one of them and you "should" be good.
If they are infrared, of which there are a few, it could be more problematic and need to be well separated visually.
John