have been wet a long time. What should I do to salvage the plant?
Repot it using fresh potting mix. If you find roots that are soft or mushy, trim them off. Water the new soil so that it is damp, like a sponge that has been wrung out. The following articles discusses care of this plant and how to get it to rebloom. https://www.gardeningknowhow.com/ornamental/flowers/poinsettia/poinsettia-care-how-do-you-take-care-of-poinsettias.htm
Any other tips for repotting?
I would not recommend a cactus soil mix.
This article will help you.
https://www.gardeningknowhow.com/ornamental/flowers/poinsettia/transplanting-poinsettia-plants.htm
Thanks to you, my poinsettia is growing well. How do I get it to change color in time for Christmas? When do I start?
This is controlled by, both, genetics, and hours of light. These must be exposed to at least 12 hours of darkness to produce floral clusters, and color change. This article will help: https://www.gardeningknowhow.com/ornamental/flowers/poinsettia/poinsettia-care-how-do-you-take-care-of-poinsettias.htm
How to prepare pointsettia for dormant period?
We have several articles that should be helpful.
https://www.gardeningknowhow.com/ornamental/flowers/poinsettia/how-to-make-poinsettia-turn-red-make-a-poinsettia-rebloom.htm
https://www.gardeningknowhow.com/.../poinsettia/poinsettia-care-how-do- you-take-care-of-poinsettias.htm
you think it will revive? The plant was superb over Christmas and for months after. I followed internet advice and pruned it back to 6 inches at the end of September. Is it still lying dormant.
It shouldn't ever look dead because you are continuing care but giving it 12 to 14 hours of darkness each evening by moving it to a closet or covering it with a box. Each morning it returns to its sunny window. Continue this till about Thanksgiving.
If it truly looks dead, leaves crispy, I think it is a lost cause. Have you been watering and fertilizing?
https://www.gardeningknowhow.com/ornamental/flowers/poinsettia/poinsettia-care-after-christmas.htm
I bought a poinsettia about a month ago, it was doing fine for a while but then the leaf tips started to turn brown. Please help
Most commonly, this can be attributed to overwatering.
This will be very common when they come from a nursery since they change environments very quickly from there to someone's home. This is very stressful, and they will not use much water during this time of adjusting.
They will need to be allowed to dry out between waterings down to about 3 inches or so before watering again.
You may find that you will need to treat with a fungicide. This article will help: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
This article will help you to care for Poinsettia indoors: https://www.gardeningknowhow.com/ornamental/flowers/poinsettia/poinsettia-care-after-christmas.htm
How far did it break up the root? If this is just the tip, then it will likely survive. If you broke it closer to the plant, then it may not survive.
Unfortunately, your image did not come through, so I can't see where the break occurs.
Just in case, I would make sure to root some cuttings from the branches. This will help you to have a replacement plant or two just in case yours dies from the injury.
These articles will help: https://www.gardeningknowhow.com/ornamental/flowers/poinsettia/broken-poinsettia-stems.htm