The downgrading operation can be done to use some apps which are not compatible with newer macOS version.

To downgrade your Mac:

1A) Create a macOS bootable installer (read more here)
OR
1B) Restart your Mac while holding Shift+Alt+Cmd+R

2) Now, restart your Mac while holding the Alt key to boot into the Book Picker and select the installer that you've just made.

3) Make a clean macOS installation (read more here).

4) You are ready to go!

Downgrading to Mac OS X 10.6 or earlier

To do so, simply grab a Mac OS X Install CD that came with your device. In case you can't find it, search for Mac OS X 10.- Install CD A---- (A---- is a model number, 10.- is a software version).

After you made sure that you have the CD, start up your Mac while holding the Alt key, select your Install CD and make a clean Mac OS X installation (read more here).