AssetDatabase.RenameAsset
static function RenameAsset(pathName: string, newName: string): string;
Parameters

pathName The path where the asset currently resides.
newName The new name which should be given to the asset.
Returns
string An empty string, if the asset has been successfully renamed, otherwise an error message.
Description

Rename an asset file.

All paths are relative to the project folder, for example: "AssetsMyTextureshello.png".