Assuming an elemental form of Magic that allows the mages to control fire, water, earth, and the wind itself (with some limited power to change states, like turning ice to steam and vice versa), as well as the ability to impose your will upon the weather. Not the climate, just the weather. Climate control will have to be done the slow way- by using the terraformed earth to convert it.
The region is a vast plain in the interiors of a peninsula, with what used to be a large river system but is now barely a single large river running through it. The rest of the land is a hot, arid desert that is mostly rock. A few oasis here and there allow the barest traces of human civilization to continue, but only barely.
The mages have a century or so to transform the region using all the power at their disposal. Their powers are stated above, but I add that they don't need to be concentrating at the spells every moment. A mage can command a cloud to rain a certain amount and then just go do something else, and return only to switch his spell off.
There are 500 or so Mages, and they need to turn this arid desert into a lush forest with greenery everywhere. Tropical plants to plant are at their disposal already, and so are animals that used to live there before the disaster that changed the climate (implied to be a magical drought that held back rains for centuries and destroyed the glaciers).
They need to change it back- but what exactly must they do? I have considered that they probably need to rebuild the glaciers that fed the old river system, which can be done by freezing magical rainwater onto the mountains. Lakes can also be built, and winds manipulated to keep down erosion of soil. Seas that the rivers drained to can supply some water through desalination (it's not difficult to use magic to separate salt from water). But now that water is obtained....well, what else should be done?