Entries can be edited and deleted in the History view. To edit an entry, tap the entry you want to edit. (If the entry is so short that it cannot be tapped, you can reach it in the agenda view.) When an entry is tapped, the edit time view opens.
In the Edit time view you can modify start time, stop time, project/task and even add a note to your entry. From the action bar, you can also delete the time slot.
Just remember that if you modify it so it “overlaps” with another entry, the other entry will be cut so there is no overlap.
If you need to move your entry to another day, you can click on the small turquoise date area and select a new date for it.