Wireless doorbell buttons can be replaced. If you have gotten to this step it is likely that you have already tried putting in new batteries. Make sure they are seated properly before you give up.
The first thing you need to do is determine what make and model of doorbell unit you have. The transmitters in the doorbell button you purchase needs to match the receiver in the chime unit. Many times the buttons will work with several models. However, they will only work with the units from a particular manufacturer.
Companies like GE, Honeywell, Broan and Westinghouse all make doorbell units. First determine the manufacturer. The best way to do this is to remove the cover plate from the chime unit. You might need a screwdriver for this. Some of them just pop off, being held in place by plastic tension clips. Be careful, don't break it. There should be a ID Plate inside the unit.
Write down the Manufacturer and the Model Number for the unit you have. It may also have a style name. Write down this as well. With this information in hand you can search for a button on the internet. It is possible that you can find one at a home supply store. They do carry some types. If you don't mind wasting a trip, you can try there first.
Once you have the correct doorbell button in hand, you can install it. Doorbell buttons are usually held in place with two screws. Remove the screws for the old button and remove it.
Check the instructions for the new doorbell button. Make sure the battery is in place before you install it. Again, two screws should be all it takes to put it on.