At any time during the initial setup or after activation you are able to edit existing records and/or add additional records. Changes to existing records may be delayed up to the length of time defined in the TTL value associated with the existing record.
For example, if you have an A record like this:
...then any change to this A record would take unto 10 minutes to propagate. The time it takes any given machine on the internet to see the new value would be passed on when the record was last cached by the systems local DNS server.
To edit a record, check the box to the left of the record and then click the "Edit" button which will be enabled at the top of the record list.