You may follow the next couple of easy steps to clear the Download Manager's data on Android 5.1.
1) Open Settings:
2) Push on Applications and then on Application Manager:
3) Please note the heading marked in red below:
4) Swipe from the right side of the screen towards the left until you see the "All" column heading:
5) Now scroll down the list of apps until you get to" Download Manager" and select it:
6) Push on the "Clear Data" button as indicated again below in Red:
Push on the "Ok" button and close Settings.