The p0sixspwn package in Cydia has been updated to untether iOS 6.1.6. Unfortunately, an update to the p0sixspwn utility has yet to be released so you would have to jailbreak via RedSn0w in order to apply the untether. It would likely be easier to update to iOS 7.0.6 and jailbreak using evasi0n7. Or you may also want to check out the SSLPatch from Ryan Petrich that closes the SSL vulnerability in iOS 6.x and 7.x. You can find the p0sixpwn package in Cydia's 'System' section. Please note that the description incorrectly reads iOS 6.1.3-6.1.5. Version 1.4 does support iOS 6.1.6.