After backup, Deactivate all the installed plugins in Plugins section as shown in the image below:
Minor releases can be updated by WordPress automatically without user permission, whereas, major updates need user permission. In order to update, click the Update button. Upgrading can be done manually as shown below:
Download the latest WordPress version from the link and the image below shows the list of WordPress versions.
Extract the downloaded ZIP file as shown in the image below:
The first and foremost thing is to download the previous version of WordPress. Follow the steps below in order to downgrade the WordPress.
Download the previous version of the latest version because installing most older versions may have some security problems. The image below shows the list of WordPress versions:
Now, extract the downloaded zip file as shown in the image below:
Now, the site is successfully downgraded to the previous version of WordPress.