Completely restringing and re-skinning a Djembe is an arduous and time consuming task, and takes a good few attempts to learn the best way to go around completing the task. Luckily, if the skin is just loose then the Djembe can be tighted by threading rope around the drum using one of several patterns which cause the vertical ropes to cross over each other. There are quite a few methods possible, some that are wonderfully decorative, but by far the most effective is Mali Weave.
This document demonstrates how to employ the Reverse Mail Weave variant.
The video below shows a demonstration of an effective Mali Weave.