First, go into Windows Explorer and find your HTC/Aces High II/offmis directory, then look for the .mis file that you want to edit.
Make a copy (backup) of the .mis file and put the copy somewhere else - you might want to revert back to the original.
Now open the Offline Mission Editor, and in the editor, choose File > Open the .mis file that you want to change.
In the right panel you will see the missions (document icon). Expand the one you want to change, then expand the route (arrow icon) you want to change.
Below all the waypoints you will see one or more small airplane icons - these are the flights.
Right-click a flight and choose "Flight Properties". In there, next to "Unit", you have a dropdown list of plane types.
If that's all you want to change, just hit OK, File > Save and exit the Mission Editor.
Bear in mind that changing the plane type may have other effects in the mission - different performance may mean that waypoint segments take a different amount of time to cover, meaning you might see some strange effects at some points.