room heater?

Just to add to what's below.
If there are no screws at the bottom they could well be spring clips. Have a look at this PDF (give it time to load) and look at the instructions on the very first page.
http://www.arcsystems.biz/manuals/%20SPL3002.PDF
Dont be suprised when the PDF opens vertically just scroll down to the bottom!

GB in Wales
March 2013

There should be some securing screws at the bottom of the cover. With these removed the cover should lift off and leave the knob controls where they are.
Have a look at this site which popped up when your heater model was googled. There are a few maintenance tips you might be interested in.
http://www.ourwanderer.org/wan29.htm

GB in Wales-CC18017873
March 2013