Hi, 
I have a hAP ac lite router which used to work fine. I recently upgraded to v6.40.4 and I am now facing several issues.
I would like to either revert back to the factory operating system on my router OR reinstall an older routerOS version to see if that helps.
I haven’t found any comprehensive step-by-step tutorial on how to do a full reinstall of the OS.
Any help would be highly appreciated.
PS. I am somewhat new to Mikrotik so please bear that in mind 
You can downgrade. Put the old rtos package and in system packages click downgrade
Thanks for your prompt response.
Where should I put the packages files? At the root folder?
Which files are needed? Should I take the full content for example of the file:
https://download2.mikrotik.com/routeros/6.37.4/[b]all_packages-x86-6.37.4.zip[/b] ?
(Downloaded from: https://mikrotik.com/download/archive)
Thanks again! 
You don’t need all packages. Just use the main package.
Just grab the version you want to roll back to (hAP AC most recent bugfix for example)
https://download2.mikrotik.com/routeros/6.39.3/routeros-mipsbe-6.39.3.npk
Drag and drop into the root of files.
Open System>Packages>Downgrade
Router will reboot and you’re downgraded.
You can do this for more or less any previous routeros but personally if you are experiencing some problems I’d go to the latest bugfix first and then go back along the bugfix line.
Thanks a lot. Will follow your comprehensive instructions and see how it goes. 
Highly appreciate your help.
Cheers
it worked. thanks!