Connect a USB drive to the USB port on the back up the Time Capsule. The USB drive will need to be at least the same size as the drive on the Time Capsule.
Open AirPort Utility on your Mac........(Finder > Applications > Utilities > AirPort Utility)
Click on the picture of the Time Capsule
Click Edit in the smaller window that appears
Click the Disks tab at the op of the next window
Click Archive Disk
This will make a complete copy of everything that is on the Time Capsule onto the USB drive
This is a very slow process. Allow a full day or two for the process to complete. You can still use your network during this time if you want.
Once the Archive has completed, it is time to erase the Time Capsule drive.
Open AirPort Utility as above
This time, click on Erase Disk
Choose the Quick Erase option.......since you will be keeping the Time Capsule for your own use
Click Erase
The Quick Erase will only take 5-10 minutes or so to complete. Once done, you can now start a new backup to the Time Capsule.
Since the Time Capsule has been erased, Time Machine will make a new complete copy of your Mac on the Time Capsule, so allow plenty of time for this to occur.
Once the complete backup of your Mac is done, subsequent "incremental" backups should only take a few minutes, since Time Machine is only backing the changes that have occurred on your Mac since the last backup.