Master pages are a huge time saver when it comes to creating a consistent look across a multi-page document. Being able to quickly apply footers and stationary page elements all at once is, without question, a necessary part of any designer’s workflow. But sometimes you’ll want to modify those elements, and thankfully that isn’t a hassle. Just [command][shift]-click ([Ctrl][Shift]-click in Windows) the page element to release it from the master page, converting it to a standard page element.
Now you can adjust the design element however you choose, keeping in mind that if you do update that particular element on the master page, it won’t update on the page you released it from. Not to worry, if you ever want to reapply that master page, simply click the master page thumbnail and drag it to that page in the Pages panel. You’ll have to manually delete the original, released master page items if you decide you don’t want them anymore.