Not quite efficient, Roldan. Apart from the compiler error when comparing a char to a String (you should use '' instead of ""), multiple String concatenations should be prevented (single line String concatenations are ok). Use a StringBuilder instead. It also has a built-in reverse operator:
Now ideally you wouldn't replace but insert the char at position 0, but that can be quite bad for performance if the last word is long - it needs to shift elements in its internal array all the time.