Last month(15-Feb-14) I have uploaded an app in to App Store with availability date 20-Feb-14 and my app status was "Waiting For Review". Yesterday(1-Mar-14) I got one issue and again I had to resubmit the app in to App Store and my app status is "Waiting For Review". My question is Should we change the availability date also when we resubmit the app?
There's no need to manage an availability date and it is separate than the review submission process and only is considered once approval is granted for the app to be sold.
You might in the future hold your patch until the first review finishes, since you may have put yourself at the end of the queue by resubmitting. If the first review gets rejected, you still need to resubmit and can bundle the fix then. If the first review gets approved, you can then decide on how long it took and whether you push the change you had before releasing or if you want to release with that issue.