Nothing is working after updating Android 2.3 to Android 4.1 I have Samsung Galaxy Ace Duos S-6802 (originally Android 2.3). I followed the steps described here to update it. In short, I installed ClockworkMod and then JellyBlastGenED.signed in my phone.

After this update, My phone is showing Android v4.1.1 under settings. The icons, fonts etc are looking very small. Also my phone is not detecting any SIM and SD card as well. The phone is having two SIM card.

I just wanted to know that how can I undo the update or fix these errors?
The files you downloaded from that site are for a Samsung Galaxy Y. Your phone is a Samsung Galaxy Ace Duos. You need to find a ROM suitable for your phone. You're lucky that installing that ROM didn't brick the phone completely.

You can't undo an update: to get back to what you had before, find a stock ROM image for your phone (probably from http://sammobile.com for a Samsung phone) and install it following the provided instructions. Alternatively, if you still want Android 4.1, look for an unofficial 4.1 ROM for your phone and install it in the same way.

See also: Where can I find stock or custom ROMs for my Android device?