Replacing a certain character pattern in a textfield when saving an entity
So, hook_field_presave, according to the documentation, "Defines custom presave behavior for this module's field types." So unless your defining a field type in your module, this is the wrong hook to use.

Instead, your gonna want to use hook_field_attach_presave. $entity_type will contain the type of entity the save is being called from (node, user, etc). $entity is the full entity object, which you can just go ahead and edit. 

For example, a very basic function like so:

function replace_multiplication_symbol_field_attach_presave($entity_type, $entity){
  if(!empty($entity-&gt;field_field_technical_description['und'][0]['value'])){
  $entity-&gt;field_field_technical_description['und'][0]['value'] = "test";
  }
}


would, whenever an enity is save, check for the field field_field_technical_description and switch it's value to "test," if it's value was not null.