Certified GKH Gardening Expert
Both overwatering or underwatering can cause yellowing leaves, as can lower light levels if the plant has been brought indoors for winter. Give the plant as much light as possible, but most plants will lose some leaves when brought indoors from outdoors.
Mandevillas need less water during the winter. Check the soil each time and water when the soil is dry to about 1 inch down. If the soil is dry and the leaves are starting to wilt, you are waiting too long to water.