If the tenancy hasn't been signed by all parties, you can contact the property manager to change the email address for you. The new email address will be a new account so you'll need to use that to sign in.
If the tenancy has been signed by all parties, then the email address associated with your tenancy cannot be changed. As long as you retain your password, you can continue to sign into your account and access the documents.
However, you have the option to change the email address associated with your StuRents account, which you can find more information on here: Change your email address