If you have a mid 2009 MacBook Pro, you can't update the OS. To update, you need a MacBook Pro (Mid 2010 or later) Model 6.2 and higher.
Jan 12, 2018 To download Mac OS X El Capitan from the App Store, follow the link: Download OS X El Capitan. On the El Capitan, click the Download button. Next, a file Install OS X El Capitan will download into the Applications folder. When the download is over, the installer will launch automatically. The new OS from Mac is here in OS X El Capitan. Apple's 12th OS release is the OS X El Capitan.Named after a rock formation in the Yosemite National Park, USA, OS X El Capitan continues and focuses on stability, performance and security in which OS X Yosemite started.
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If you are careful about what you download and where you search on the Internet, your computer is useable without the latest security updates. My last computer had a 10.6 partition which I used every night. That was after El Capitan was released, so the OS was 5 versions behind. By being careful, I never had any problems.
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I would make sure that you had at least 2 separate backups on 2 separate drives in case you have problems. That will allow you to recover.