buffy time travel

im looking for post chosen post not fade away fics where buffy maybe with dawn and someone else go back to season 2,6 or 7 to try and change things

There are several stories by dreamweaver which involve time travel that you may want to check out. The ones that are closest to your criteria are:

Her Way
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=30191
After Spike burns up in the Hellmouth, Buffy demands that the Oracles send her back in time.

The sequel to Her Way is Changes
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=30476
This is fluff, a sequel to "Her Way", a romp through all the changes everybody has to go through once Buffy and Spike claim each other. Do read that first, otherwise things might not make sense.

Learning The Dance
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=30134
An accidental wish makes Buffy switch places with her future self.

Pinch Hitter
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=35422
Buffy vanishes after stopping Adam and the Powers That Be bring over another Buffy from a parallel dimension to take her place.

The sequel to Pinch Hitter is Revenant
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=35617
This is a follow-up to 'Pinch Hitter', the other side of the coin, the original young Buffy of the end of Season 4 faced with the fallout and a suddenly returned Spike.

Something Redux
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=31684
A year after Spike's death in the Hellmouth, Buffy is flung back in time.

Some Others:

Prologue to Life/I Know You by Slaymesoftly
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=9206
A response to a challenge at the Bloodshedverse where Buffy is returned to her season II body with all her post Season VII memories. She is there due to a spell Willow performs to try to help Buffy get over mourning for Spike.

Sequels are I Know You, II and Knowing and Watching
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=26228
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=34178

There is a very good time travel fic on Elysian Fields that should fit in with what you want called The Butterfly Effect by cousinjean
http://dark-solace.org/elysian/viewstory.php?sid=421&warning=NC-17
Five months after the events of "Chosen," a still grieving Buffy must follow a time-traveling demon back to 1997 (circa Season 2) Sunnydale to prevent him from destroying the timeline--and try not to destroy it herself in the process.

Here are a couple fics that involved Spike traveling back in time. You asked for Buffy to be the time-traveler, but if you have the time, these are definitely worth a look.

And Here We Go Round Again by Peta
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=7077
Spike takes a trip back in time to School Hard to see how possible it could be to make a Buffy,free from the trauma of first love with Angel,fall in love with him. Will this be the first one of Spike's plans to succeed?

His Way by dreamweaver
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=30151
After 'Not Fade Away' Spike finds himself resurrected yet again and sent back in time by the PTB to fix things for Buffy.

Okay, hopefully this should keep you busy for awhile! Hope you enjoy!

Not quite what you are asking for, but wonderful in every way, Eurydice is such a brilliant author and these two fics are on my all time favourite list.

Legions of True Hearts (Time travel)
http://www.loveslastglimpse.com/legions.htm
Post graduation, a restless Buffy goes to London to escape the memories of Angel, only to lose herself in unexpected dreams. But is William real, or is he a distraction? Is he, possibly...both?
Buffy/William.
Rated NC-17

And the sequel (no time travel)

A Symphony of Echoes
http://www.loveslastglimpse.com/echoes.html
Sequel to Legions of True Hearts. A confused Buffy is trying to get on with her life, but Spike's promise to stand by her haunts the Slayer. Her past collides with her future, forcing her to decide which---and who---she can trust.
Buffy/Spike
Rated NC-17

Hope you enjoy!