Split stack

1. Click the Add raster layer and select the file C:/OSGeo4W64/geodata/raster/s2/Subset_S2A_MSIL2A_20170619T.tif.
2. After clicking Open, the raster layer appears on the QGIS canvas.
3. Go to Raster --> Conversion --> Translate(Convert format).

Specify the multiband file as Input Layer. Select path and name of output file. Click and type the following additional options into the textbox: -b 1 -ot UInt16 File:Qgis split stack.jpg