It's relatively easy. Just be aware this will render the current seal invalid immediately and it's impossible to roll this change back.
You just need to log on to your account, go to the Management menu, under Secure Seal Configuration. You must choose which website are you going to do this for.
On Step 1, you'll see a page like this:
You can re-generate (or recreate) your Secure Seal by choosing a different technology that it's currently being used and then doing this process again and choose your previous technology. E.g.: PHP to ASP, then ASP back to PHP.
On this same screen, there's an easier way. Hit CTRL+SHIFT while right-clicking on the seal technology you're using.
Then, you must re-upload the new seal to your server.