You have 2 activations for two computers. So you simply install and activate using your serial number.
If you decide to uninstall CS5 off your iMac, don't forget to deactivate your by clicking on Help > Deactivate.
Thank you for your response. I had tried to install however I was receiving a message that there was an error. I was able to click on the "?" and change a setting on my mac.
Oh yes I forgot, the security setting needs to to be set to allow software from Everywhere. CS5 was released before Apple's Gatekeeper.
Next, download the Application Manager update. Adobe - Adobe Application Manager : For Macintosh : Adobe Application Manager
Once that is in you can update CS5 via Help > Updates.