The plant looked nice in the store, but now the leaves are turning brown. Help!

i bought a Mandevilla plant, which is a climbing vine with pink flowers on it. It is in a pot with a small trellis. It was looking beautiful in the store. i brought it home about 3 weeks ago and i have the pot on the deck. It gets full sun all day, and the temperatures have been in the 70’s and 80’s. I had been watering it generously but the leaves started turning brown so i started cutting back on the water. The leaves are still not looking their best – brown on the edges. Does anybody know what could be the problem? it’s supposed to get full sun, so that’s appropriate. i don’t want to kill it!!