The Office is a special District in Craftville which permits you to manage your Empire's Personnel.

From the Office, you can fire one of the reserve Managers and hire another one with a different Mine specialization at the cost of 1 Crystal Crystal-icon:

This can also be accomplished by Retraining a manager within the Managers tab for the same cost once the Office is unlocked.

Retrain The Manager

20 Managers with the same specialization can also be exchanged for 1 production Expert at a cost of 10 Crystals:

Exchange For Expert