On your account page, click the Domains tab, and click the link under the Name Server column for the domain you want to change. The link will say something like "Dynadot DNS: [IP]". At the top, set the record type to A, and enter the IP you'd like www.yourdomain.com to point to. Underneath, there's a bit list of empty fields. Set the first empty box to "subdomain", set the droppdown to an A record, nd the third text box to the IP you'd like to host subdomain.yourdomain.com at. Click "Use Dynadot DNS" at the bottom, and your changes should propagate shortly.
Select "My Domains" from the left-side menu bar and click "Manage Domains" in the drop-down.
Check the box next to your domain name(s) and click on the "Action" button.
Choose "DNS Settings" from the "Action" list.
On the DNS Settings page, select the "Dynadot DNS" setting from the top drop-down menu.
In the "Domain Record (optional)" section, select "A" from the drop-down menu and then enter your IP address in the "IP Address or Target Host" box.
In the "Subdomain Records (optional)" section, enter "www" as the subdomain, select "A" from the drop-down menu and then enter your IP address in the "IP Address or Target Host" box.
Press the "Save Settings" button to save your changes.
Please be aware it might take some time for your settings to propagate.