There's Back Track & it's sequel Step Forward by AuthoressNebula at Elysian Fields, although it's both Buffy and Spike plus two other scoobies who go back. The summary for both are:-
Back Track - Set the night following a girl's death and a trip through a portal (Season 7, Get It Done), four Scoobies meet in the kitchen for what they think will be a sad, uneventful night of coffee and memories. It probably would've been, too. That is, if they hadn't wanted to go back and change the past so desperately that they decided to try time travel.
Step Forward - Many things have been changed. Lives that should've ended the previous year continue into this one. Relationships that had been destroyed the first time around are now solid and sure. The one cloud that hangs above their heads, however, is the First. Will it appear? If it doesn't, what other things will they face? And if it does, will they win?
At Nocturnal Light there's also Fix You by Enigmaticblue. The summary is:-
Desperate times call for desperate measures where the First is involved. In order to prevent the end of the world, Buffy asks Willow to do a spell that's supposed to fix everything, and Spike goes along for the ride.
Also at Elysian Fields you could try Fixing The Factors by ImmortallySpuffy. Although they don't go back in time to another season they do jump back in time in a particular episode over and over again in a timeloop. The summary is:-
Set during The Yoko Factor, something has happened to make Buffy and Spike repeat the same day over and over. Will they figure out they're in this together or keep working against each other? Will they be able to fix the time loop or are they doomed to repeat the day for eternity? And just how many visits from a jealous Riley and Angel will Buffy be able to stand?
Enjoy!!
Sarah
