The only way to "downgrade" is to restore a backup of the database and files which was made before the upgrade took place.
Note: Any new posts/members/threads/any other changes made after the upgrade will be lost.
You could try reverting out of date templates, which would probably fix your errors. However it will cause the loss of any customisations to the template.
See the "Template Changes" section of this thread (
https://vborg.vbsupport.ru/showthread.php?t=177013) if you want to try and update your skin yourself.