There are multiple possibilities. The two easiest ways are:
1. Use the s.mart Songbook Online Editor. There is a online edit mode, which allows you to edit your song comfortable in the browser of your computer. For more information see: http://www.smartchord.de/song/edit
2. Synchronize your songs with your cloud. During synchronization, the songs are stored as text files in your cloud. From your PC, you can access and edit these files in the cloud. With the next synchronization, the changes are imported into the songbook.