In Calendar view on the Day by Day tab, hover your mouse over the bottom border of the attraction's time block and then click and drag the border up or down to adjust the duration of your visit.
Alternatively, you can click on the attraction block and then the Edit prompt to set the arrival and departure time.
If you need help resolving scheduling conflicts, click on the notification icon on the top navigation bar (it turns red when a conflict arises). Then, click the prompt for help fixing the issue and select the option for the software to recalculate the trip for the destination where the conflict occurs.