Unfortunately, downgrading Ignition is much trickier than upgrading Ignition. This is because gateways are backwards compatible, but not forward compatible. For example, you are able to restore a 7.5.12 backup or project into a 7.7.1 gateway, but cannot restore a 7.7.1 backup into a 7.5.12 gateway.
1) Before attempting to downgrade, make sure that you have a Gateway Backup that was taken from the version you want to downgrade to or an older version.
On older versions, trying to restore a backup from a newer version into a Gateway on an older version will cause your Gateway to stop and trying to start it again will give you a NET HELPMSG 3547
On 7.6.7+ versions of Ignition, trying to restore a backup from a newer version will result in the following error:
2) Unactivate your license. Go to the System>Licensing page and click "Unactivate License". If you have internet access, confirm that you want to Unactivate. If you don't have internet access, you'll have to perform and offline Unactivation.
4) Download the correct installer by selecting the desired version from the dropdown list of our Archived Downloads and install as normal.