The latest maintenance release (MR15) for SFOS v17.5 is now available through the MySophos licensing portal.
To install, you can download the firmware from the Licensing Portal. Please refer to Sophos XG Firewall: How to move to a different firmware version.
Is it possible to upgrade to v18 with MR15?
Yes - This will be possible with V18.0 MR4
v18 MR4 has not been released yet, when is the release date expected?