Gardeners have differing techniques for overwintering Mandevilla, though in general, don't expect the plant to remain all that attractive since they won't get enough light and ambient humidity to thrive while inside. Ample leaf drop is to be expected and will be a normal reaction to the drastic change in conditions, so be prepared for a few weeks (possibly sporadically all winter) of leaf yellowing/browning and shedding, likely from the oldest/lowest regions of the stems first. Once it is put outside again next May or June, regrowth will fill it back in again.

Put the plant in as much bright direct light as possible (next to a sunny window or under moderately-high-strength growth lights) and consider using a room humidifier, especially if several plants share a room for overwintering. (Misting foliage and using pebble trays to boost humidity, which are commonplace recommendations, are not that effective.) An outbreak of spider mites, mealybugs, or possibly whitefly would not be unusual, so periodically inspect stem junctions and the undersides of foliage to catch a pest outbreak before it becomes extensive. You will have limited treatment options indoors since few insecticides are labeled for inside use. Hosing off the foliage thoroughly (upper and lower surfaces) and letting it drip-dry before bringing it indoors may help knock down any minor pest populations that went undetected.

You should be able to cut the plant back if it's rangy and/or needs to be removed from a support for climbing stems. Normally pruning triggers growth in plants, but since it would be signaled to go dormant due to shortening daylength and the change in conditions, it might not regrow much until spring, which would be fine. Mandevilla has milky, latex-like sap, so try to do any trimming outdoors with enough time for the sap to stop oozing from the cuts before bringing it in, if any sticky drip residues would be a problem.

Miri