If the failed slot is not master-capable, first:
- Disconnect the failed slot from the stack.
- Enable master-capability by typing REBOOT AS MASTER-CAPABLE at the pending-AAA login prompt.
Once master-capability is enabled:
- Login to the switch and install the license key.
- Type "configure stacking slot <slot number> master-capability off" to disable master-capability and if neccessary connect the slot back to the stack.
- Reboot the slot so that it rejoins the stack.