I bought Lightroom 3 and installed it on my MacBook Pro via a disc. Later I upgraded via web to Lightroom 5. My question is how can I install a copy of Lightroom 5 on my brand new iMac?

Hi Wesleys,

You need to download Lightroom 5.2 on your new iMac.

Download link for Lightroom 5.2 :

Adobe - Lightroom : For Macintosh : Adobe Photoshop Lightroom 5.2

While installation, it will ask you to enter the serial number for Lightroom 5 and then the previous version serial number in order to validate it.

Also, once your product is installed and activated you can upgrade Lightroom 5.2 to its latest version Lightroom 5.7.1

Hope this helps you to fix the issue. Please feel free to contact us back for any further assistance.

Thanks

~Mrinmay