Anybody help me with advice on how to change my catalytic 1998 Honda Accord EX

 

It can be a bit of work to get it off. You often need to take tin snips to get the heat shield cover off but you might get lucky. Then soak all the bolts you are going to remove in PB blaster or something in the hopes you will be able to unbolt them. Your next problem is getting the O2 sensor unscrewed from the old catalytic convertor to plug it into the new on. You will need a special too for removing O2 sensors, you can often borrow these from the local parts store. If it's truly old, you might want to just buy a new O2 sensor so you don't have to mess with it and to have a fresh sensor. There are three bolts on the front and back of the convertor to remove it.

I'm not sure what else to offer...but here's a place to get the convertor cheaply:

Here's a video that walks through what I described above.

Good luck.