Apple yesterday released the iOS 13.5.1 update to patch the 0-day vulnerability that was being used by Unc0ver for their jailbreak. If you value your jailbroken iPhone or iPad though, you should immediately turn off the automatic update feature of iOS 13 and then jailbreak your iPhone again.
Pwn20wnd of Unc0ver team posted a message on Twitter confirming that there is a bug in the latest versions of Unc0ver which does the opposite of user preference for auto updates. This means, even if you have auto-update turned off, your iPhone could automatically end up updating itself.
There’s a bug in the latest versions of #unc0ver that causes the jailbreak to operate with the opposite of the user preference for disabling auto updates. You should turn off that option and re-jailbreak now to prevent your device from auto-updating to 13.5.1.
— @Pwn20wnd (@Pwn20wnd) June 2, 2020
The workaround to this problem until the Unc0ver team releases a new update is to turn off the ‘Disable Auto Updates’ option in Unc0ver settings manually and then re-jailbreak your device again to prevent it from updating to iOS 13.5.1. You can also use one of the many jailbreak tweaks like OTAdisabler to prevent your iPhone from automatically updating to iOS 13.5.1.
Apple is still signing iOS 13.5 so even if your iPhone has updated itself, you can still downgrade back to it. However, the company will likely stop signing iOS 13.5 sooner than later so make sure to downgrade your iPhone as soon as possible. Given that iOS 13.5.1 does not bring any usability improvements or other new features, there really is no need to update your iPhone to it as well.