You can change your master password from Enpass Settings–> Security–> Change Master Password.
If sync is enabled on your device, your master password will also be changed on the supported cloud along with the device.
NOTE: Changing Master Password on one device will not automatically update the new Master Password on other devices. Next time when you open Enpass on another device, it will open by your old password. In case, if cloud sync is enabled then the app will prompt a sync error and ask for the new Master Password. As soon as you enter it, Enpass will automatically update your old password with the new one.