You don't. It will mess things up. If you have 3rd party plug-ins it is best to install from disk if you have them.
Generally speaking, you want to visit the web site of the maker of each plug-in and get their newest installer, compatible with Photoshop CS6, if possible.
For those older plug-ins, if you have no way of getting a new installer and installing them into Photoshop CS6 directly, then you'll want to locate just the files that were installed into your earlier version of Photoshop, and copy those files to the equivalent locations in the new Photoshop.
This is not always trivial to do this, as often plug-in installers put multiple files in multiple places, so if you're not completely comfortable with dealing with files and your operating system, I suggest against it.
But if you have some technical prowess, you can often locate files in the Photoshop plug-ins subfolder as well as the main Photoshop installation folder by matching up their Modification Dates.
Fortunately, Photoshop CS6 seems to be quite compatible with plug-ins that worked with Photoshop CS5, - BUT - you'll need to pay special attention to make sure you keep 32 bit plug-ins separate from 64 bit plug-ins, and see that each type goes into the proper place in Photoshop CS6. Note that while there's a 32 and 64 bit Photoshop CS6 for Windows, for the Macintosh it ONLY comes in 64 bits, so if you have 32 bit plug-ins working with Photoshop CS5, they can't work with Photoshop CS6.
Do NOT attempt to shortcut this process by setting the Extra Plug-ins folder in the new Photoshop to point to the files in the old Photoshop's installation area. That will bring you nothing but trouble.
If you'd like more specific help, please feel free to provide more system specifics, as well as the names of the plug-ins you want to transfer.
Good answer Noel.
This FAQ might help too:
If you do copy plugins, you should only copy third party plugins, not the included Adobe plugins. It is recommended that you re-install 3rd party plugins to ensure that all the libraries end up in the right place. Some plugins have been redesigned for CS6 and some that previously were only for 32 bit have been redesigned for 64 bit. Check with the plugin provider.
Don't copy all the plugins from a previous version or point to an old version's plugin folder because you will create duplicate plugins. Adobe included plugins are in a different folder now so you are pointing to them twice, which causes issues. Duplicated plugins from a previous version are the known cause of a file type menu getting out of sync in the save dialog.