Hello,
The lower the number, the higher the priority. If there is only one MX record, then the number doesn't really matter.
There was a DNS bug in version 1.20.0, which was fixed in 1.20.1. 1.20.2 is out now, so you can just click "Upgrade" in the License / Updates. I'll be send out the update command soon, so it should be done automatically anyway.
John