You can change the status of a task or a lead from active to done by following below steps:
1. Go to task or lead screen and the specific task or lead as you want
2. You can see the tick button (V) on the left of Owner column or on the right upper corner of detail view of that task/lead
3. Click on the (V) button, the task/lead will be moved from active screen to historic screen

Go to historic view of task or lead, now you can see that task/lead displayed