Changing raster cell values within polygon using QGIS GUI

First, you need to convert your polygon to a raster. Create an integer field filled with "1" for the purpose of the conversion. (raster > conversion > rasterize)

Then, you can use the raster calculator to substract 1.5 where your polygon exists.

 yourdem@1 - ((poltoras@1 = 1) * 1.5 ) 

1. make copy of raster (raster2)

2. Use Raster / Conversion / Rasterize

   • select you polygon layer, make field with -9999 or some value which is not in your raster2
   • as target use your copy raster2 raster/grid
   • Keep existing size and resolution

3. Use Raster / Raster Calculator...

   • expression should be like this: ("raster2" = -9999)*("raster1"-1.5) + ("raster2" != -9999)*"raster2"