You can use Explode 
on the Advanced Editing toolbar to separate a selected multipart feature into its individual, component features. This would be useful if you needed to alter the attributes of one of the elements in a multipart feature.
There are also geoprocessing tools available to merge and separate features. The Merge and Multipart To Singlepart geoprocessing tools work on all features in a layer rather than just the selected features.
- Click the Edit tool
on the Editor toolbar.
- Select the multipart feature you want to separate into individual features.
- Click Explode on the Advanced Editing toolbar.
The parts of the multipart feature become independent features, with each one being assigned identical attribute values.