Resource utilization: changing work hours per day changes the workload defined in the task

Resource utilization: changing work hours per day changes the workload defined in the task

Hi,
I planned my tasks adding fixed hours for each resource, let's say: John works 5 hours and Julia works 3 hours to complete a task.
If I see that they are overallocated I thought to change the hours per day but WITHOUT changind the estimaged working hours.
How can I fix ghe estimated hours? Estimated hours should change just in the task editing