If we can LEARN a phobia, we can UNLEARN a phobia.
Systematic desensitisation is a behavioural therapy designed to gradually reduce phobic anxiety through classical conditioning.
Counterconditioning occurs - this the learning of a different response to the stimulus (e.g they learn to associate the phobic stimulus with relaxation instead of anxiety).
It's also based on the idea of reciprocal inhibition is when you can't feel two emotions at once (so anxiety and relaxation can't be felt at the same time).