I'm not familiar with the particular layout of your engine, but with any spark plug change you want to make sure you don't damage the ignition-wire-to-endclip connection. If you simply pull on the ignition wire, you're sure to damage the wire end.

They make a special tool shaped like a plier called a spark plug boot remover. In the absence of this tool the first thing I'd do is twist the boot back and forth to break the baked-on seal. Then grab the boot - ideally right where the clip goes on the spark plug end under the boot - and pull the boot off.

Two things you should do for a proper spark plug change is to lightly coat the threads of the new plugs with an antisieze compound and to butter the inside of the spark plug boot with silicone grease for easy removal next time. Both are available at any good auto supply place.

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95 E320 Cabriolet, 169K