As the fall winds to a close in November, water less and less until you stop watering altogether. This will help the begonias go into dormancy for the winter. You may do this with either garden or pot-grown begonias. If you live in a hot climate, forced dormancy isn't necessary, as the begonias will survive the winter outdoors. Trim the begonias back for the winter. They'll begin growing again in early spring. Use a shovel to dig them out with a ball of dirt around them. Set them in a sunny window in a shed or garage for up to a week to let them dry out and cure. Store the tubers in open flats lined with peat moss or sand in a cool, dry place for the rest of the winter. Digging the tubers out of the ground is not necessary if you live in a place where the ground never freezes. Plant them as you originally did, helping them sprout indoors in early spring and then transplanting them outdoors after the last frost has passed and the ground has thawed.

TL;DR in Vietnamese:
Dần dần tưới ít nước lại. Đào các củ lên. Trồng lại củ vào đầu mùa xuân.