One option (free) would be to use the Archive function in AirPort Utility.
Simply connect a USB drive (formatted for Mac) to the USB port on the Time Capsule
Click the Archive button and a complete copy of everything on the Time Capsule will be made
This might take 4-6 hours or more depending on how much data that you have on the Time Capsule
The downside to using the Archive function is that you must make a new complete copy of the Time Capsule each time that you do this.
Another option would be use a utility like Carbon Copy Cloner to automatically back up the data on the Time Capsule to a USB drive attached to the Time Capsule on a schedule that you choose......something like once a day, or once a week, for example.
The advantage of doing it this way would be that once you have the first backup of the Time Capsule, Carbon Copy Cloner will only back up any changes that you have made since the last backup, so subsequent backups will only take a few minutes.