First go into the day you'd like to change. While in the day view, simply tap on the (...) icon in the bottom right hand corner of the thumbnail. This will bring up some options for that second.
On an iOS device
Have in mind that if you no longer have the source video of your snippet in your Camera Roll, you'll have limited editing capabilities. For example, you won't be able to change the portion of the video you selected to create your snippet from or to change the snippet's duration.
On an Android device
Please keep in mind these actions are irreversible. The option to edit snippets is only available in the iOS app at the moment.