It is possible to replace those gages... The most important step will be to make sure the system is off and full dis-pressurized before you try to remove the old gage. @PeterLoron is correct that the brass screw in thingy on the top (technical term) is for releasing pressure. You can find replacement valves in a variety of on-line stores such as: here, here, or here. When you remove your thermometer from the line, the back should look like below:
These valves come in a variety of sizes and different thread patterns and in a few cases, if when you look in the back you cannot see the hex shape to get a wrench on in the back, a few are even soldered in (you may need some help to replace this type if you're not familiar with soldering, they can be touchy). You may decide to replace it with a temp/pressure gage as well.