There are many different versions of OSX, the instructions below should assist with most recent versions.
Click the Apple icon at the top left of your Mac’s main menu bar, and choose “Software Update” or click directly on App store.
Within the App Store is a tab labeled “Updates” – click on that tab.
You will see any available updates for your operating system and any installed applications.
Select “Update All” and your system will update, and display a list of recently installed updates.