Once a campaign has been launched, it is not possible to change its optimisation goal.
Each campaign collects data and learns based on the selected optimisation goal. Changing the goal while the campaign is running would reset this learning and could negatively impact performance.
For example, a campaign with a CPA goal is optimised towards generating conversions. If the goal were changed to CPC, the campaign would optimise towards clicks and would need to restart the learning process.
If you want to use a different optimisation goal, pause or stop the existing campaign and create a new one with the desired goal.