Dunno about retensioning it I guess it wouldn't be that hard once you pull the belt out, they aren't overly engineered pieces of technology.
Pulling them out is fairly simple. From memory (if it's wrong shoot me)
Remove the plastic sill panel and door rubber from both the front and rear door, or atleast lift them up enough to free the plastic panel that runs up the B pillar. They just clip in but be careful not to damage them.
Remove the both sections of B pillar panel and door seal rubber, the top section may only need to be lifted in a few locations to allow a hand up the pillar.
Remove the bottom bolt holding the belt in and remove any screws/bolts holding the mechanism in and pull out.
I guess you might be able to tension the retractor without completely removing it but it might be fiddly.