You have to export the gerbers twice and then mix the resulting gerber files.
1). Make sure you save your project.
2). Export gerbers as normal.
3). Go the routing menu and select all vias.
4). Go to the edit menu and select delete minus.
5). Export the new gerbers to a new location (I always use a subfolder called “tented”)
6). Overwrite the .gbs .gts .gbp .gtp .gbo .gto files from the original export with the ones from the “tented” export.
Now check the results in something like gerbv or online with Aisler or Easyeda etc to make sure it looks correct.