Folder Sync?

I do not recommend downgrading due to changed code signing certificate. But maybe previous installer getting re-signed with it.

You could first try to disable folder sync and then set it up again. This should be your first step. Because downgrading has a lot more steps and potential pitfalls.

But if you want to downgrade, the installers of previous versions are in the linked https://app-updates.agilebits.com/product_history/OPW6 next to the release date of an update.

You can try and run the installer over the current installed version, but it likely says that a newer version is already installed.

You can try to just uninstall the current version via Settings > Apps and then run the installer again. It could be, that the installer of the older version still says a newer version was installed.

If this is the case, then you have to delete the 1Password Folder in AppData/Local (as described https://support.1password.com/uninstall-1password/), which also includes https://support.1password.com/backups/.

Doing this would reset all settings, including Folder Sync, so you would have to set it up again anyways.

You might want to make a copy of the 1Password folder before deleting it, so you could copy it back if something is not as expected. Also make sure that you have a Backup of your Standalone Vault(s) outside of the 1Password folder.

Be sure you have access to your https://support.1password.com/secret-key/ if you use an 1Password membership, your https://support.1password.com/two-factor-authentication/ (if enabled) and, of course, your 1Password to unlock your vault(s).