How to modify body of a node
There's no such hook as hook_node_save(), you're probably looking for hook_node_presave()


  Act on a node being inserted or updated.


Also, the nody body is a field so you have to treat it as such when assigning values:

function mymodule_node_presave($node) {
  $node-&gt;body[LANGUAGE_NONE][0]['value'] = 'test string';
}