Maureen you should be able to just cut the "head" off to the height you want and place it straight back in the pot or ground. From everything I have been told on another forum (don't want to name it in case I get in strife) Agave's are happy with this. Other ppl I have been told allow them to "dry out" for about 5 days before re-planting.
If you want the link send me an email and I'll happy respond with the link.
Cheers