To install manually.

Right click on 7DTD in Steam library.
Click on properties.
Click on local files and then Browse.
Click on Data folder.
CLick on Config Folder.
Then Drop the File named Items into the config folder, it will aske to replace items, simple click yes.
If you want to change game back to normal, simple verify game file integrity in game file properties on steam. This will take game back to normal.