Some nodes, and most blocks, contain special formatting elements, in the form of embedded HTML or scripts. If you are going to edit a node, take care not to blitz somebody else's hard work by not paying attention to the details that don't show in text.

The edit window that appears for the body of the block is a formatting editor, but it doesn't always protect you from making a mess. This is why backing up the SOURCE contents of the block is so important. As a last resort, the original writer of the block should have kept the original source, so don't ever hesitate to ask.

After you have backed up the contents, take a look at the block in the default (non-SOURCE) mode. If it looks pretty much just like the block looks when displayed to an ordinary visitor to the site, it's probably mostly text, and can be easily edited. If not, however, you may be staring at some serious concealed HTML construction, and you should tread carefully.

Select the SOURCE button in the block edit controls, and look at the contents. If you can easily discern the location and structure of the text area you want to edit, go for it (since, of course, you have backed up the data, right?). If not, perhaps you need to ask someone with more HTML experience for a little help. On this team, that should not be a problem.