How to save OctoMap data everytime?

Hi, I'm getting a 3D map using the OctoMap server package. But As the vehicle moves in this mapping, the map updates itself to the exact location where the vehicle is moving. What I want to do is not delete the previous data of the map until the vehicle is gone from the starting position like 2d mapping. How can ı do this? Could you please help me.  This is the 3D map I get using PointCloud2.