How to upgrade kernel to 4.13 (which is in the new image)?
or
If this is not possible, then how to reinstall HiveOS and keep all settings?
The latest image 0.5-45 is using the Linux 4.13 kernel.
Unless you have installed something directly on the Rig HiveOS has all the settings on the Server which it sends to the Rig.
All you have to do is download the latest image from here: https://hiveos.farm/install
Install it to an USB Strick or SSD/HDD drive.
Boot to it and let it update itself to 0.5-46
You will then be prompted for Rig # and password
Enter them. Settings will be sent to the Rig and mining will start running.