Refer to this post from Jolla for instructions. You can just flash the binaries over the top of your existing setup. No loss of anything, all going well. I guess back up could be sensible.
Note down the list further there is a suggestion to add a restriction to the fastboot command if flashing fails. I had to do this, despite using USB2. I think it depends how new your fastboot is.
Finally, for me, as suggested above these binaries are rock solid. No freezing. Just better mobile data and a battery indicator that works.