Be prepared to be frustrated, angry, and want to find a donor tube on craigslist so you can ditch with one altogether.
There's lots of good tuts on adjusting convergence out there. I suggest reading up to get a good understanding of the concepts.
Always exercise caution and care when doing anything on a live monitor. IT IS HIGH VOLTAGE! That said, always keep one hand in your pocket. Always be careful when reaching around behind the tube. You can touch a live part without getting shocked (altho I recommend staying away from them altogether), but when you touch a live part and a grounded part (like any of the metal shields, casings, the monitor brackets), the current from the live part will make a b-line for ground through whatever is conductive (in this case, you). Keeping one hand in your pocket at all times ensures you aren't giving current a path through your heart.
Warnings aside, if your problem is only in the corner of the screen, then you can probably stay away from the convergence rings, and just use a "convergence strip" to correct it, or at least get it better.
If you look at the back of the monitor, you should see 3 or 4 white strips that are hot glued or silicone-rubbered to the back of the tube. They extend down between the tube and the yoke collar. At the end of each of those strips is a small piece of metal, which has a small magnetic effect on the color guns. You may be able to simply remove the one that is in that corner of the tube and reposition it to correct the mis-convergence. Or you may find that there isn't one there...possibly whatever was securing it dried up and it fell out (check the bottom of the cab). You may also adjust the one that is there but find it does not fully solve the problem, in which case you can add a convergence strip.
Unfortunately, convergence strips do not exist for sale, but fortunately, they are easy to make. Good overall info on adjusting convergence, ending with an explanation on how to make DIY convergence strips: http://www.arcaderepairtips.com/2014/02/28/monitor-convergence-tips/
Another alternative for convergence strips is taking them from a donor tube. Someone giving away a free CRT tv near you? It'll only cost you the headache of dealing with the typical craigslist'er, gas, and time to open up the TV and steal its convergence strips. Oh, and then taking the TV to the next free local recycling event.
Hope this helps. If re-adjusting the existing or adding a convergence strip doesn't help, post back.
TL;DR version: You issue can likely be resolved with convergence strips. You might be able to re-adjust the one that's there. If that doesn't help, they can be made easily or stolen from a donor tube. Google for more info.