To refill you need to get the glass bowl out. Use a knife or small scraper under the edge of the bowl and gently go round prizing it up. Be very gentle though, the glass can crack easily.
Once you get it moving it should come out quite easily. The next job is to unscrew the brass cap on the back of the bowl. These can be very stubborn. you could try tapping around it to free it up, or some heat. Not a naked flame though.
All being well it will come apart and you'll have a puddle on your bench . I should have said do the above over a bowl.
Refilling can be done with compass oil, although I never found any! Baby oil, a bit thick but it does work, or lamp oil.
Either fill the glass bowl and while holding upright put it back together and hope you don't get any air bubbles, or put everything in a large bowl of the oil and assemble in the oil. No bubbles. A small bubble won't matter either way you do it.
Hope that helps