CS5 download

You install the same way you did the 1st time... put your disc in the drive and enter your serial number when asked, or download the installer and enter your serial number when asked

kglad links in reply #1 here https://forums.adobe.com/thread/2509712 to download program installers

You should always mention your operating system... since you didn't, notes for Mac and Windows

CS6 and earlier programs have not been tested and will not be updated for Mac El Capitan or later operating systems

-which means that you try to use CS6 and earlier at YOUR risk of having problems due to Apple updates

-Apple did not maintain backward compatibility with old programs in El Capitan or Sierra

--Adobe has a workaround to Apple's problem of not maintaining backward compatibility

--READ HERE https://forums.adobe.com/thread/2301916 for an Adobe solution to installing old programs, including a link to installing the old Java runtime that is required

--Also read for more information https://forums.adobe.com/thread/2458382

Two ideas that MAY work to install and/or run old programs in Windows 10

-RIGHT click the program icon or EXE and select a compatibility mode in the pop up option window

-or Run as Administrator http://forums.adobe.com/thread/969395 to assign FULL Windows permissions may help... this says Encore, but is sometimes required for ALL Adobe programs (this is NOT the same as using an Administrator account) so follow the instructions, substituting the name and location of your specific program for Encore