Airline Club

After clicking the airplane tab, click on the model u own, on the right u should see "Inventory" with the aircraft u own for that model, click on the airplane icon in inventory, it show show details of that aircraft 

If it's too old u can simply click "replace" button in the details and it should replace ur old aircraft with a new one w/o having to reassign

Hope this helps!

Extremely helpful thank you! Is there any way to increase your maintenance budget or refurbish aircraft to increase there reliability or condition?