If you want to modify a Contract that you already published, no problem. Just find that Contract in the Contracts dashboard and click New Version.
Make any changes you want in the editor (including uploading a new version of the PDF if it's a PDF Contract). Then click Publish when you are ready for the modification to go live. The new version of the Contract appears immediately on your Legal Center.
Don't forget! If you turn on the Contains updates that require acceptance toggle on a clickwrap Contract, your existing users will be asked to accept the modified Contract the next time they log in (as long as you have implemented the login widget).