You sure can. Your Single Grade Membership gives you access to one textbook at a time during your 365 day subscription. To change textbooks, go to your account, then click Membership>Subscriptions>Change Textbook to switch textbooks.
You might find this quick tutorial helpful too!
You are, however, allowed only one textbook change per month. If you need to make a second change, please send an email to info@nicolethemathlady.com.