Replace gear shifter with one with gear number display
There are two things that you need to check to be sure that the shifter will work. The shifter needs to have the same number of gears as your bike. If you have 7 gears on the back, then don't go buying a 10-speed shifter. The second thing is that the pull-ratio of the shifter has to match the derailleur. This is why not all shifters with the same number of gears are interchangeable. This information can be found online or you can update your question with details of the derailleur on your bike and the shifter that you're looking at.