How downgrade macOS to previous versions safely? I working on macOS Mojave 10.14.5

I am new to macOS and I don't know much about installing different versions. I can't dare to uninstall the current as I am afraid to lose my file and installed packages. 

how can I safely downgrade to 10.11 or 10.10?

Updates: info about my MacBook: 

MacBook Pro (Retina, 13-inch, Late 2013)

The reason why I want this is that I want to install wifi USB adapter but it seems that it's only compatible with os 10.11 or below that. Having this wireless adapter working is so much important to me though. 

Adapter details: 
EDUP 802.11n USB Wireless 
Model : EP-N8553
There is no downgrade option available which let's you 'go back to a previous macOS version'. Most Mac Apps automatically convert your Photo, Mail or iTunes files to a newer database version. So you won't be able to import Mails out-of-the-box from Mail (Mojave) into Mail (El Capitan).


You will need to backup your data, first 
Erase your disk or volume using Disk Utility
Reinstall from macOS Recovery or USB drive.
Move back your data from backup


There's no need to reinstall your system due to USB adapter issues. It rather sounds like a incompatilbity issue. Your USB adapter is too old. Try using your USB adapter on another friends, collegues computer for further testing.

The only guaranteed step is to erase the system, reinstall a clean OS and only migrate / import data that was backed up to the lower version of the OS or older. You generally can load an older backup on a newer system, you generally cannot load a newer backup level on an older system.