Navigate to the Admin section at the top of the page.
From there, click Team Settings on the left-hand side of the page. Scroll down to the section Dates & Time. Click on the default start & end times and choose suitable times from the drop-down list.
If you want to change start and end times on specific tasks only, scroll down to the section Projects & Tasks.
Make sure the box 'Enable editing of start & end times on individual days' is checked.
You will then be able to edit start & end times when creating or editing tasks.