First find the crystal oscillator by referring to the previous table and description of its physical characteristics. Be careful when you remove the oscillator.
Most people just use a normal soldering iron, and are fine.
Use copper wick to soak up the solder from all four pins, and pop out the proper oscillator. There was recently one report of a guy who damaged his IIsi board doing this. But that was the only incident I have ever heard of, and lots and lots of people have done this. They melt the solder, and have an electric pump to suck out the solder while you swirl the pin from the oscillator around to get all the solder out. After you have done all 4, if you have done a good job, the oscillator just pops out.