Yup! You can do it from the settings tab in the mobile apps but it's easiest from the web app. Here's the instructions for changing your password on the web.
Sign in to your Avocado account at avocado.io/login.
Visit the settings tab (the last icon that looks like two people)
Click "change shared password" button to update it.
IMPORTANT: You will change the password for both of you at the same time. Don't forget to tell your boo what the new password is!
And, lastly, if you're not able to remember your password at all to even start this process, you can always request to reset it.