Accidentally I upgrade Joomla version from 3.6 to 3.9 and it was a big mistake, as it destructed all alignment on home page. Need to know, how to downgrade to 3.6 back?
You should restore a back up that you took of your site prior to the accidental upgrade. :)
There has been 31 releases between 3.6 and 3.9.9 where something like 3.7 alone introduced 700 improvements including structural changes to the database for things like Custom Fields. 3.8 and 3.9 have been preparing Joomla for compatibility with Joomla 4 with some minor releases having between 50 and 300 packaged updates/improvements each.
There is no automated way in Joomla to roll back updates that i am aware of, so to be able to manually restore nearly 3 years worth of updates without causing further problems would be close to impossible IMHO.
Unless you have a back up I would say you need to follow any advice provided to get your now 3.9x environment working correctly. And take more back ups.
I agree with Irata but there may be a simpler solution without having to roll back to Joomla 3.6.
Depending on which template you are using, and depending on whether there were custom changes to the template CSS file or a separate custom CSS file, you may be able to fix the alignment problems by reverting back to the original CSS file that came with the template and/or reapplying the custom CSS changes or custom CSS file, if this is able to be recovered from a backup or similar.
If there is no backup, then I recommend trying to fix the alignment problems via a custom CSS file or similar and go forwards rather than backwards which could cause more problems than it fixes.
Always run a backup before making changes so you can undo the changes if needed.