Spring would be best to repot your plant.
Leave it in it's existing pot to overwinter.
For a mandevilla that is still quite small, work your way gradually up to that size, by moving it to a pot 2 inches larger every time its roots begin to protrude from the drainage holes of the old one. Plants grown in containers that are too big for them are much more likely to contract root rot than those in tight quarters. Make sure the pot has adequate drainage holes.