Trouble-shoot one iMac diagnose hardware issues and offer an estimate for a complete solution.
Upgrade Software
From the Apple menu, choose Software Update while your computer is connected to the Internet. Software Update will check for new versions of your Apple software. If it finds any, the updates will appear in the Software Update window. Click the Install button to download and install the software.
You can configure Software Update to automatically check for updates either daily, weekly, or monthly.