In most configurations, once an objective has been confirmed an individual cannot modify their own objective.
Managers and other senior people can re-open an objective to allow modifications. This effectively puts the objective back into draft mode.
Once the modifications have been made, the objective can be confirmed again.
Select Change the original objective.
The objective will re-open in draft mode. Note that the progress updates will not be shown in draft mode, but will re-appear once confirmed.
Make your changes and save it as draft or confirmed.
The objectives panel will reload showing your changes.