Multipart Split
This QGIS plugin Multipart Split allows the user to, during edition, split the active layer’s selected multipart features into single part features. Unlike the “Multipart to single part” from the Vector > Geometry Tools menu, the plugin only splits the selected elements, and does not need to create new files.
Installation The plugin is available in the QGIS official repository, therefore the installation is done in the Manage and Install Plugins (Plugins > Manage and Install plugins…).