Yes, you can!
To upgrade:
1) Make sure you are logged into your 1st Domains account
2) Click on 'Manage Domains and Services'
3) Click on the Domain Name you wish to manage
4) Click on 'Upgrade Email Hosting'
5) Tick the box next to your domain name, select a plan to upgrade to from the drop down and click continue
6) Verify your Invoice billing information is correct, click continue
7) Check the requested services are correct, confirm the period and select your payment method and click on process order.
Note: The hosting services expiry date will be aligned with the expiry date of the domain name.
For downgrading:
This can be done when you are renewing the services, we suggest doing this close to expiry.
You'll also need to make sure you have not got mailboxes using up more storage than you want to downgrade to.
When you are renewing a domain, on the checkout page you'll see a drop down to choose a different plan, select the plan you want and choose a payment method and click process order.