Go to https://www.zotero.org/download/ and click on Other Versions arrow under Installation help to reveal version 6. Just download and install that. That won't effect your data folder, only the application will be replaced.
But, before you do that, which plugin? Many plugins have been replaced by new ones that have the same functionality. You will have to do a new installation of the new plugins; an update of the old would not help in that case. Someone would be able to point you to the replacement here.
Zotfile has been replaced by two new plugins, zotmoov and attanger. Some of zotfile's file naming abilities have been added to stock Zotero, so that also helps.
There was some work done to update zutilo for 7 (https://github.com/wshanks/Zutilo/issues/248), so it may still show up (unlike zotfile, which won't be updated). But some of the functionality is now in the Actions & Tags plugin.
Upgrade Storage
But, before you do that, which plugin? Many plugins have been replaced by new ones that have the same functionality. You will have to do a new installation of the new plugins; an update of the old would not help in that case. Someone would be able to point you to the replacement here.
As for plugins, I need Zutilo. Also ZotFile would be nice to have, although not crucial.
There was some work done to update zutilo for 7 (https://github.com/wshanks/Zutilo/issues/248), so it may still show up (unlike zotfile, which won't be updated). But some of the functionality is now in the Actions & Tags plugin.
See the discussion here: https://forums.zotero.org/discussion/108833/zutilo-in-zotero-7-which-of-its-functions-are-most-essential
Others may say more.