On your currently running system, in the GUI, navigate to System → General → Save Config and save the downloaded .db file.
Safely shut down your FreeNAS system, and unplug all internal drives.
Remove the boot SSD and set it aside, in case things don't work out.
Connect your new boot USB stick, power up, and boot from a FreeNAS installation DVD.
Install FreeNAS to the USB stick.
When the install is completed, shut down your system (or interrupt the reboot at the BIOS screen) and plug your data drives back in.
Start up your system from the USB stick. You may need to find the IP address from the console.
In the GUI, navigate to System → General → Upload Config and provide the path to the .db file you saved in step 1.
Once uploaded, your FreeNAS system should reboot once more, and this time power up with your pool present. Verify that all files are accessible and there are no errors or faults.