By using the Line 6 Updater and the flash files they're providing (and by telling the Updater explicitely that you want to perform an offline update). I wrote this little, err... "essay" on the L6 forum, should still be pretty much the same and very likely pretty much identical with the HX One:
After posting the same advice over and over and over throughout the last years (and for several times just during the last hours), I decided to post this here, so I can just link to it (it's also missing in any official Line 6 documentation, at least in a direct, straight ahead form). So, your up...
line6.com
Even with my rather layman technical understanding, several things seem to point to issues with the L6 server during whatever online update procedures. And even if it's something totally different, going for a forced offline update brought their seemingly bricked units back to life for many people.
Note: Even if it's mentioned in the link above, I can't stress that enough; In case you're on Windows, really make sure there's no traces of the HX Librarian software running in the background. Those don't cooperare well with the Line 6 Updater, resulting in your unit not being recognized.