Unfortunately, I realized I couldn't switch due to some restrictions from mobstac leaving me with the option of re-subscribing or my site gets deleted from mobstac's server.
I decided to re-subscribe to see if I can switch when I have an active plan and it worked. My subscription will expire tomorrow but I have been able to successfully downgrade my account to the free plan yesterday.
So, if somehow you still want to be using mobstac but don't want to keep paying for their service, follow the steps below in switching to the free plan.
==> Log in to your account at mobstac.com
==> From your site's mobstac dashboard, go to Content > Sections and make sure u don't have more than one section. If you have more than one section, delete all.
==> Then, go to "Account", click on "change plan"
==> Right under starter plan, click the "switch now" button
==> If successful, you will see : "Switched to Starter. Your account settings have been updated. Your site has been now switched to the Starter plan".
If you want to stop using mobstac completely, you can delete the CName created for your mobstac site from your DNS dashboard and delete your mobstac site from your mobstac dashboard.
I hope this helps.