If the objective status is "Approved", you can not edit it since it is locked.
Please ask your manager to revert the status to "Draft" so that it will be editable.
For any other question on the objective management / the performance review, please refer to the handbook from the following URL firstly:
https://shiseido.box.com/s/4ejn577rndrty9r7vgj17ts8s7ovvfdk