Depends on your Mac. What version OSX does the Update button in About this Mac offer?
I would not recommend Sierra for CS5 and you might need to install legacy Java. Download Java for OS X 2015-001
You can upgrade in place, no need for a reinstall.
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First of all, you need to check is your system qualify for Yosemite update or not. If it does then you can update your machine to the latest available Mac os update and try using the Photoshop.
If in case you face any issues using the Photoshop after the update, then you may need to reinstall the Photoshop application.
How do I tell if my system is eligible for Yosemite update or not?
You mean, what does the update button say once I've installed Yosemite? I haven't installed it yet. I'm still running on mtn lion.
Thank you for any further help.