public virtual void SetMetadata( 
   string metadataXml
)

MapMember mapMember = map.GetLayersAsFlattenedList().FirstOrDefault(); //Search for only layers/tables here if needed.
if (mapMember == null) return;

//Gets whether or not the MapMember stores its own metadata or uses metadata retrieved
//from its source. This method must be called on the MCT. Use QueuedTask.Run
bool doesUseSourceMetadata = mapMember.GetUseSourceMetadata();

//Sets whether or not the MapMember will use its own metadata or the metadata from
//its underyling source (if it has one). This method must be called on the MCT.
//Use QueuedTask.Run
mapMember.SetUseSourceMetadata(true);

//Does the MapMember supports metadata
var supportsMetadata = mapMember.SupportsMetadata;

//Get MapMember metadata
var metadatstring = mapMember.GetMetadata();
//TODO:Make edits to metadata using the retrieved mapMetadata string.

//Set the modified metadata back to the mapmember (layer, table..)
if (mapMember.GetCanEditMetadata())
  mapMember.SetMetadata(metadatstring);