Here is how to reduce the resolution of raster file in QGIS:
1. Open QGIS (with GRASS).
2. Find “Raster->Projections->Warp” tool and then click to open the window.
3. Check the “batch” mode
4. Select the directory where your DEM files are and specify the output directory.
5. You should choose “EPSG:4326″(WGS84) as source and target SRS.
6. Select a re-sampling method.
7. Click the small pencil icon besides the command box. Add “-tr” option in the last parameter.
The original spatial resolution of ASTER GDEM is 30 meters or 0.000277778 degree ( 1 arc-second). The spatial resolution should be the same unit as given SRS, i.e. in “degrees”. Please choose a proper value that is suitable of your case. ( Maybe 100m or larger)
8. Run the warp tool.