I'm going to assume youve got a non cup model? If so you'll need to remove the tension from the spring on the aux belt tensioner, most people do this using a big jubilee clamp around said spring and tightening it up, once you've done this it's a case of removing the belt and unbolt in the old alternator, fitting new one and doing the above in the opposite order.
One thing of note is that the aux belt tensioner/belt assembly is a known weak link on the f4r engines and I believe Renault recommend changing every 36k miles or 3 years, whichever comes around first. Thus is one of the more common u dealing reasons for cambelt failure as the aux belt will wrap around the bottom pulley.
