Hi.
Before any upgrade please take a full backup of your important data.
There is no "Do it so and it will work!" guide.
I can describe how I updated my bare-metal systems from 6.0.X to 6.1.1 (including a Gen8 MS)
- Backup your data
- Disable any SSD cache
- Shut down your server
- Unplug your boot stick or micro-sd from the server and open it's grub.cfg
- Write down the important modifications and entries like serial no., macs, sataportmap, etc.
- You can either write the new synoboot.img to your old boot device or use a new one
- Download the 1.02a2 image for DS3615XS from here: https://mega.nz/#F!BtFQ2DgC!JgomNP3X8V9EuwxL4TXbng!Fwki3A4B
- Use a tool like OSFMount to mount the synoboot.img (15MB partition) and uncheck the "read-only" option, open it's the grub.cfg
- Check the VID & PID of your new device or (if you intend to use your old boot medium) use the old values
- Fill in the entries mentioned above (vid, pid, serial, etc.)
- Save the image and unmount it
- Write the image to your desired boot medium with Win32Diskimager, plug it into to your server and let it boot
- Log in to DSM and go to the control panel -> Updates & Recovery
- Download the official PAT file from Synology: https://global.download.synology.com/download/DSM/release/6.1.1/15101/DSM_DS3615xs_15101.pat
- Select "Manual update" and upload the previously downloaded PAT
- Confirm the update and let the server reboot (this can take a little bit more time as usual)
- If anything went smooth you should be able to re-login into DSM
- Apply actual Update 4 and after another reboot everything should be finished