When you first created an Apple ID, you picked an email address and a password. But iTunes lets you at any time change the password you use for the iTunes Store, and App Store from iPhone, iPod touch, and iPad. This tutorial explains how to change your iTunes password through the iTunes Store, and how this will affect all other devices you have connected to this Apple ID.
Open iTunes, and follow these steps to change your password:
The next time iTunes asks you for a password (when you want to access your account, make a purchase, rate an item in the iTunes Store, etc.), just remember to enter your new password. Since you can have up to 5 computers authorized with iTunes, each of these versions of iTunes will also use the new password, if they are connected to the Apple ID in question.
Likewise, since your Apple ID is tied not only to iTunes, but also your iPhone, iPod touch, and iPad, remember to use your new password when one of these devices asks for it (assuming these are set to use the Apple ID whose password you just changed).
Tip: to change your Apple ID / iTunes password from your iPhone or iPod touch, go to Settings > Store > View Account. Follow these exact steps to change your iTunes password from an iPad as well. Since all these installed instances of iTunes, and other Apple devices, are all connected, you just need to remember that if you change your Apple account password in one place, all iTunes and devices connected to that Apple ID will be affected (and you'll need to use your new password for all of them).