knotty is correct, the MRs are cumulative.
With a HA config, do the following:
- Backup the BES SQL DB
- Upgrade the Standby Instance to MR7 (this will upgrade the DB)
- Fail over
- Upgrade the new Standby instance to MR7
- Fail back (optional)
- Upgrade the BES remote components such as BES Router, BES Attachment Service etc.