Earlier today, Apple released iOS 6.1.2 where shortly after, planetbeing confirmed evasi0n works on iOS 6.1.2 but we would need to wait for it to be updated. The evad3rs have officially updated evasi0n for Mac, Windows, and Linux to officially support the iOS 6.1.2 firmware.
If you are already jailbroken on iOS 6.1 or below, you'll need to update via iTunes and restore to iOS 6.1.2 and repeat the jailbreak process. Keep in mind that users have had issues with over-the-air (OTA) updates followed by a jailbreak, so make sure you update via iTunes before attempting to jailbreak iOS 6.1.2 using evasi0n.
If you need help with the jailbreak process itself, you can follow our guide linked below. Hit the source link to download the newest version of evasi0n.