This is something that can happen with any app, during an update, and isn't a problem that's specifically related to Hipstamatic.
What most often causes this situation is when you've tried to update an app and there's some kind of network hiccup during the download. That's what's essentially caused the download to become 'stuck' as it is now.
There are a couple of things you can try:
1) Do a reboot of your phone. Sometimes starting the phone again will trigger the download to start again
2) Switch networks. If you're on wi-fi, try switching to 3G or vice versa.
3) Sync your phone with your computer. Again, like rebooting, sometimes the syncing process will trigger the download to finish.
Unfortunately, those tricks are sort of hit-and-miss in terms of success rate. As you can see in some threads from around the net, this is a problem that doesn't seem to have a solid/reliable solution:
The only *sure* way to get the app fully unstuck is to reinstall it. That's not something I normally recommend. In the case of reinstalling Hipstamatic, that also deletes all the images in your Recent Prints gallery. They should still be in your Camera Roll, but always make sure you know you've got copies before you reinstall Hipstamatic. Your paks, of course, can all be restored for free. I can provide you with instructions for reinstalling if none of the other suggestions above work.
Sorry we can't be of more help on this one. But hopefully something above will help get things resolved for you.