When you update your selected Validators, it takes two Eras (Era = 24hr) to process, meaning that in the third Era after submitting the change, you will see it reflected in your Nominations list.
Current Era: the task is queued to be broadcast to the Pallet during the rollover to the next Era
2nd Era: the task is broadcast to the Pallet to update your selected Validators
3rd Era: the task is processed during the rollover to the next Era, and your Nominations will reflect your current selection