As we know that previously the Mobile Substrate app was reported error by number of users who jailbroke their 64-Bit based devices (like iPhone 5s and iPad Air) using Evasion iOS 7 jailbreak tool.
The situation forced an iOS developer to release a Mobile Substrate fix app while waiting for the Saurik to taking care of the problem, and he's done with it. Therefore the man released Substrate 0.9.5000. Here is his statement via his official Twitter.
Substrate 0.9.5000 has been released, with support for iOS 7 and ARM64. (For extensions to work on ARM64, they must be recompiled to ARM64.)
— Jay Freeman (saurik) (@saurik) December 31, 2013
And yes, you are recommended to install this update so the app -- as one of the most wanted Cydia tweaks who were reported errors after Evasi0n iOS 7 jailbreak installed -- can serve you like it used to.
Surely you want to install this update, therefore just launch Cydia on your iPhone or iPad Air then find the latest Cydia Substrate, when you found it then install the tweak.
Anyway, have you jailbroken your device? You haven't? Then you should considering the best things you would have with. Therefore I suggest you to learn on how to untethered iOS 7 jailbreak tutorials.