Your device must already be jailbroken with Chimera to re-jailbreak.
Credits
wh1te4ever (original developer)
alfiecg24 (stable kernel exploit)
felix-pb (kernel exploit)
Samuel Groß (CVE-2020-9802, WebKit exploit)
CoolStar and all Chimera jailbreak developers
JakeBlair420 and all TotallyNotSpyware developers
qwertyoruiop (Mach-O loader with +jitMemCpy)
forcequit (site and user experience improvements)
If any contributors are missing, please let me know.
Troubleshooting
Manually reloading the web app • While in the web app, press and hold the Top/Side button until you reach the "slide to power off" screen, then press and hold the Home button until you reach the Home Screen.
"A problem repeatedly occurred..." • If you're in Safari and this message appears while you're trying to rejailbreak, reload the page. If you've added the page to your Home Screen, and solely wish to jailbreak from there, see above to reload the web app, or restart your device.
Device reboots to Apple logo after "Jailbreaking..." for a few seconds • Leave your device powered on and unlocked for ~30-60 seconds before retrying again, it may take multiple attempts to succeed.
Failed, retry? • The re-jailbreak is failing for some reason. One possible cause for this can be that your device is already jailbroken. If retrying doesn't help, refresh the website, reload the web app, or restart your device.
Device doesn't reflect updates to site • If you're using the web app, see earlier instructions to manually reload the web app, and/or reboot your device. If you're not, I probably didn't update the AppCache manifest. Go to Settings > Safari > Clear History and Website Data and it'll get new data from the server.
Unable to remove problematic tweaks • If you can't remove any tweaks due to your device crashing when trying to reboot to a jailbroken state, you can disable tweak injection temporarily by setting the NVRAM boot-args variable to -safe-mode using either an SSH RAM disk, or the irecovery command line tool on a computer, with your device connected in Recovery mode. Please visit the GitHub page for further instructions.
JavaScript Disabled • Go to Settings > Safari > Advanced, and toggle JavaScript on. If you're using a 3rd party web browser, check your browser's settings to enable JavaScript, or use the Safari instead.
If you have a problem not mentioned here, please file a GitHub issue.