Use Boot Camp Assistant while booted up in OS X. You can find it in the Applications/Utilities folder.
If your system has had that boot camp partition for 6 years of use, then you may find out that you can't recover that space with Boot Camp assistant, as it may report errors, or just tell you it can't remove the partition.
It happens.
If you see those kind of errors when you try the Boot Camp tip, then your only real choice at that point is to boot to another drive, erase your boot drive, removing all partitions in the process, then restore OS X from your backup.
Hopefully you won't need to do that, and the Boot Camp tip will work the first time to recover that partition space.