I agree with advice that it is dangerous - the problem is the back of the CRT display, which has potentially lethal voltages exposed if you open up the top part of the case. Even if the computer has been disconnected from power for a long time, it can have large residual voltages sitting on the CRT. You need someone who knows how to safely discharge a CRT before opening that part of the case. Anyone trained in repairing old CRT TVs should be able to handle this safely. Disposing of the CRT safely is also tricky, as it is a vacuum tube, so it will implode if broken.
I've never tried to open the top half of an iMac G3, but I have opened the bottom half to get access to components like the hard drive. There are two substantially different body designs, easily distinguished based on the style of optical drive. The earlier models have a tray-loading optical drive and the later ones have a slot-loading optical drive.
The tray-loading models have a removable module which includes the CPU, main logic board, hard drive and optical drive. Removing this is relatively easy, but it can be tricky getting it back together again with the cables sitting in their channels, so it fits well.
The slot-loading models need a little more effort to open, with the lower part of the outer body unclipping to gain access to the logic board, hard drive and optical drive.
I'm sure there are general take-apart instructions on
www.ifixit.com, at least to get as far as the logic board. They probably don't go as far as instructing how to access the CRT, so you may need to explore that on your own (with help for the safety aspects).