Following the release of iOS 13.5.1 last week, Apple has stopped signing iOS 13.5. This means that you can no longer downgrade your iPhone to iOS 13.5 if you had updated it to the latest release of iOS.
iOS 13.5.1 was a security fix from Apple to patch the 0-day vulnerability that was used to jailbreak iOS 13.5. If your iPhone is running iOS 13.5.1, the only way you can downgrade it back to iOS 13.5 is if you have saved the required SHSH blobs. Otherwise, there’s no way for you to downgrade your device to iOS 13.5 and then jailbreak it. Unc0ver is unlikely to be updated to support iOS 13.5.1. Checkra1n might get updated to support iOS 13.5.1 since it relies on a bootrom exploit, though it only supports iPhone X and older devices, and it is only available for Mac and Linux.
With iOS 14 announcement around the corner, developers are now likely going to wait to see what the next version of iOS brings to the table and focus on jailbreaking it.
If your iPhone is already jailbroken and running iOS 13.5, I’d recommend you to stick to that setup. Make sure to check out some of the best jailbreak tweaks and sources to further enhance your jailbreak experience.