Not, pun intended, an absolute necessity to know but the Halyard knot is simple and very handy whilst your busy reversing your halyards.

Its not a Knot designed to come undone so if its been in your halyard for a while you may find you need to cut the old one off.

But the new one is just a matter of a couple of wraps and putting the end through the loop before tightening down.

You can find an example of how to do it here.