To rejuvenate the plant, you can cut it back by up to ½ but wait until late winter or early spring. I need to do the same to mine. The extra foliage will protect the crown from frost, snow & extreme cold. Right now I'm getting a lot of snow so I'm definitely not going out there now! Happy Holidays!

I'd dead head them and if they are likely to suffer from wind rock reduce them to about 1ft so that there is some protection over the winter. If you have some where frost free then you could use the prunings as cuttings.

If it is a shrubby/woody one then it will re-sprout from the stems in spring, if you need to then, prune further to where you want it. The removed material will make excellent cuttings.
If it is a true herbaceous one I cut down to 1ft then remove all the dead stems when the growth starts again in spring.