Hi
There is no UIR bit in Register B; it's 5th bit is UIE which means-
Update-Ended Interrupt Enable (UIE): This bit is a read/write bit that enables the update-end flag (UF) bit in Register C to assert IRQ. The RESET pin going low or the SET bit going high clears the UIE bit. The internal functions of the device do not affect the UIE bit, but is cleared to 0 on RESET.