How do i change a UV light in a water sterilizer unit?

I took the plate off the bottom and the electrical plug is there , but how do i get the light tube out of the unit itself? there is a knurled knob that i cant seem to turn, shall i force it?Thanx.How do i change a UV light in a water sterilizer unit?
Don't force it. If you break it then you're looking at how many hundreds of dollars to replace it? Do what I do.... call a water servicing company and pay them to change it. I do it once a year and it cost me $80. Money well spent to have clean water.