Hi KL Klein,
There are two things that you can do:
1. Write on A word document (if on windows) or an equivalent and then copy it on Hubpages.. .And if you do any changes on the Hub, update it back on this document on your PC. or laptop..... Otherwise, if you are one of those who write on Hubpages directly, then I suggest copy the content and paste on some document on your computer.
2. Once the hub has been published, right click on your hub directly and click on "Save as" and the entire page is saved on your computer..
Hope this helps.