If you press all four buttons of your Nike HRM it will reset. Next time you go to "RUN" mode, it will show "HRM LINK" error. You will now need to "hard reset" the unit.

Nike made a late change to add a "hard reset" to the chest strap. To re-link (re-initialise) the watch you'll need to open the battery door on the chest strap, and use a paper clip (or the equivalent) to push the small gold button in the battery area. There should be a small sticker in the battery compartment that points to the reset button.

The re-initialisation steps for the Nike C10 and CV10 digital HRM's is:

(1) reset the watch
(2) open the battery door on the chest strap
(3) move the watch to RUN mode
(4) press the reset button in the chest strap battery area
(5) put the battery back in the chest strap (if it's not already in - it's possible to push the button without removing the battery)
(6) hold both of the electrode to create a signal (probably not necessary - it usually links immediately).

Eventually, Nike will either eliminate the hard reset or update the manual to include this info.