Place the object that you want to change on the canvas. Set up its properties (Font, style, point size etc.) to the defaults that you want. Now drag and drop that object back into the Object Library. It will ask you for a new name (You can give it a new name or give it the same name and it will overwrite the current object). Conversely you can create a new library tab to hold all of your new objects (so that you can keep the defaults for whatever reason). To create a new object library tab, on the far right side of the Library Tab there is an icon that looks like a paragraph with a downward pointing arrow head. Click on that icon and a new menu will appear. Click on Add Group. A dialog asking you to enter a new name for your group will appear. Enter a name and click OK. Your new group tab will appear at the bottom of the list of groups in the Library tab. Now you can drag and drop any objects that you want onto this Tab. Note that you can also have multiple objects represented by a single object (like Address block).
Hope that helps
