There should be 4 screws at 2, 5, 7 and 11 o-clock (shown in photo) possibly with white or grey cover caps on them. Underneath these may be self tapping screws in which case they will come straight out. OR they could be machine screws with nuts underneath in which case you will need to access below either via a cover panel or by removing a display unit at the helm or something.
I went through similar 10+ years ago - I found that it was costly for the manufacturer to service the unit. I tried to do it myself - I purchased mineral oil and a syringe but with mediocre results (I still had an air bubble). I ended up simply replacing the unit - I would likely have purchased from a) Fisheries, b) Defender or c) Ebay at that time
I just checked and assuming it is the 135 Defender sell it for $431 and Atlantic Rigging have it for $364