I know of a couple fics where Spike is the one who travels back to season 6, can't really think of one where it's Buffy...
The only thing I can really think of is It Came Upon The Midnight Clear by Holly, but Buffy doesn't travel back in time in that one, it's more like she gets visited by three ghosts (past, present and future) and she sees glimpses of a possible future.
I know of some where Buffy travels back in time, but she travels back into an earlier season (season 2, 4, 5 etc.), I can post those if you want.
After some searching, I've found these:
There's Do Over by maryperk, in that fic Buffy, Willow, Xander, and Dawn travel back to season 6.
http://dark-solace.org/elysian/viewstory.php?sid=4019Forever Chosen by kittyfajitas (Both Spike and Buffy travel back to season 6) - WIP
Summary: Spuffy. Time travel. Need I say more? Spike and Buffy find out that sometimes even Champions can choose their own destiny.
http://dark-solace.org/elysian/viewstory.php?sid=4035Learning The Dance by dreamweaver
An accidental wish makes Buffy switch places with her future self
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=30134Learning to Love by anzu_brief (It's a WIP, only 3 chapters posted, hasn't been updated since last year)
Summary: Spike is dead, and the world is ending. The armies of the First has defeated humanity. To restore balance Buffy must go back in time to her old body and prevent Willow from resurrecting her. She will fight Glory, close the portal and remain dead for good this time. It's a good plan. Sadly, plans rarely go as planned, especially when there is a vengeance demon involved. When Buffy is forced to face life instead of death, and carry on with all the ghosts from her life of seven years ago, what is she going to do? Is this a second chance for happiness? Or is the Slayer always condemned to fight and die alone?
http://dark-solace.org/elysian/viewstory.php?sid=3930