how do i replace the fan belt on my vauxhall corsa?

I assume you mean alternator belt? Slacken off bolts securing alternator, swing alternator towards engine, install belt, adjust tension by swinging alternator away from engine. Tighten alternator securing bolts. Tension should permit 1/2" play on longest run of belt between pulleys.
Best if there are two people - one to hold alternator away from engine to tension belt, while the other tightens the securing bolts.

ChicksFan
September 2009