You mean the bulb is burnt? If so, you have to remove instrument cluster and replace the bulb. There are procedures how to remove the cluster in Technical Manual, e.g.
https://www.sl113.org/wiki/Electrical/RheostatSince it is a relatively demanding process, you may want to think what to do once you are there, e.g.:
1. Replace all the bulbs (this is what I would do), including tacho and speedometer
2. Consider LED replacements (lots of posts about it)
3. Connections cleaning, rheostat maintenance