In default the servers are in the order you made the servers account in MesNews. Within the server the groups are in the order you subscribed. To change the order of groups and servers you use the menu Group\Display order of the groups.

Left you see the list of server: when you click on a server you will be shown in the right pane the groups you are subscribed to.

The arrows up and down helps you to move a group or server up or down. The two buttons to the right under the arrows lets you arrange the groups automatically: in alphabetic order, grouping by synchronization (first fully synchronized, then groups from which you download the subjects, then groups you only collect the number of new messages, then not synchronized groups)

Once validated the servers and groups are rearranged (in tree of messages or in serversbar)

Remark : You can also rearrange the servers or groups in the servers or group window: click on server/group name and move the server/group while holding your mouse button down to the desired place.