There are 3 base cases that you need to consider: 1) an empty list, 2) a list with one element, and 3) a list with two or more elements. If you consider these three cases individually, the function is pretty easy to define:
change :: Ord a => [a] -> [a]
change [] = []
change (x:[]) = [x]
change (x1:x2:xs)
| x1 > x2 = x2 : change (x1 : xs)
| otherwise = x1 : change (x2 : xs)
EDIT As @kqr has pointed out, you can rearrange this into
change :: Ord a => [a] -> [a]
change (x1:x2:xs)
| x1 > x2 = x2 : change (x1 : xs)
| otherwise = x1 : change (x2 : xs)
change other = other