It's a bit tricky but here are details on each step you'll need to do. To set the A record, go ahead and:
1. First, log into your Domain Manager by following the instruction of the "Domain Manager Login" section of our support docs.
2. Once you're logged in, click on the domain name you want to edit.
3. Under Nameservers, click on "Set Nameservers".
4. In the pop-up window, check "I have specific nameservers for my domains".
5. Clear any nameservers already entered.
5. The nameservers you should use instead depends on when you registered your domain - find out the values based on your domain registration dates here.
6. Just copy the appropriate nameservers from that page to the domain manager. Click on OK to save, and refresh the page.
7. When the nameservers change, you'll see some information under Total DNS, and a link that says "Launch" - click that link to edit your DNS records.
8. The A record of your domain should be listed as the first record on the list. In the Actions columns on the left, click on the pencil/notepad icon to edit the record (make sure you click on the icon for the same line that the A record is on).
9. Finally, use the information provided to your by your new host to fill out the "Host Name" and "Points To IP Address". The field labeled "TTL" should stay in the default "1 Hour" value. Click on OK to save the new record.
After that your new A record should go live after a short delay.