The OnePlus One, out of the box is a diamond in the rough. The software version out of the box is 22R. You can easily upgrade to 25R using the build in Over The Air (OTA) software. Note this an incremental update. It requires you to have a full working version. installed first.
The next higher versions are 25R, 30O and 33R I have seen how incremental installs fail before, so I am cautious of going that route. A user posted a link to a full version of 38R, all 452MB of it. I quickly downloaded and prepared to flash. To my surprise, my TWRP kernel is gone. Seems that I have accidentally upgraded from 22R to 25R and part of the process upgraded the recovery manager. (To disable, Settings, Developer Options, uncheck upgrade bootloader.)
Using the OnePlus One tool kit from XDA, I managed to reinstall TWRP recovery manager. This time I remembered to uncheck the update. Installing the recovery manager is a slow process. Be patient and wait for it to self restart before continuing.
Flashed the full update in TWRP, reboot and everything is still there.