The THRUST force is greater than the DRAG force. This aircraft is speeding up.

How can the pilot make it travel at a steady speed?

The pilot slows down the engine. Now the two forces are the same size.

The forces are balanced. The aircraft travels at a steady speed.

How can the pilot make the aircraft go slower?

The pilot slows down the engine further.

Now the DRAG force is bigger than the THRUST force.

The forces are unbalanced, and the aircraft starts to slow down.