You can do either of two things in my experience. If the geranium is in a pot or can be potted, you can take it inside and keep it in a sunny window, where it should be treated as an ordinary house plant. It will continue to grow and even flower. Another option is to leave it in the pot, cut it back to about 8" for a short variety or 2' for a tall variety, and keep it in a cool/cold basement or crawl space where it won't freeze. Water it once in early January.
A third option, that hasn't worked for me, is to pull the geranium and store it in a paper bag in a cool, dark space for the winter.
Last year I put half a dozen plants in the basement for overwintering I watered them occasionally and they did well. First warm day in April I put them out and a few hours later happened to look at them and they had all blanched completely white. I guess I shocked them. One eventually recovered. Anyway, not sure why that happened, but be aware.
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