Updating the element size of voxel mesh

I have meshed a geometry using voxel mesh with element size 0.5mm.  I have seperated the mesh for design and non design space for topo optimisation. But now I feel the element size is too small and want to only change the element size to say for example 5mm (voxel mesh). How can I do it. I don't want to redo the whole model again.?( Like seperated the design and non design space ). Please guide me.