You will need to create an account in iStall first. See here for more details on creating an account.
Once you have created an account and verified your information, sign in to your iStall account settings. This can be done by clicking Sign-In on the top right hand corner. If you are having trouble signing in, see here for more details on resetting your password.
Skip this step if you clicked directly on the (account) link above. Once you are logged in, under the settings icon, click on My Account, or simply click the button below.
Under My Account, click on My Parking Registrations, or just click the button below.
You will now be looking at a list of all of your active iStall parking registrations. To update the license plate you wish to have registered on the lot, simply click "change". (see below)
You will now be prompted either to directly change your license plate, or alternately, you will be shown a list of paid monthly recurring parking registrations. At this point, either type in your current license plate, or select "Edit" next to the license plate you wish to change. Once you have edited the license plate, click Save.
And that's it, you're done!
And just in case you need a shortcut, feel free to simply click the buttons to view and edit your iStall Account or your iStall registrations right from here.