Task can be edit in different view.
- Tabular view
Go to Tasks > Tasks list
click on edit button on right side of table and the form will be visible with existing values. update task and click on save.
- Card view
Go to Tasks > Tasks Cards
Hover on a specific task so the pencil icon will be visible just click on it to edit task.
- You can also edit and update the tasks for specific contact or lead. Go to overview of any of the contact or lead > click on Task tab > click on edit button. A form will be visible to you with existing values, do your modifications and click on Save button.