This option is used to update the part being edited in the Parts Editor so that it is consistent with the number of pins on its gate symbols, and the number of pads on its footprints.
Locating this option
Available from: Library Manager option > Edit button > Parts tab > Edit menu > Update Part option
How to Use This Option
On selection, the libraries will be searched for the symbols and footprints that the part uses. You will be informed if any of these have had a change of pin or pad count, and given the option to automatically update the part.
If you are editing a part and you change one of the symbols or footprint, then this option will be automatically invoked.