At the end of the Voyager series, Admiral Janeway travels 26 years into the past to quicken Voyager's travel home. She sacrifices herself in the process. Does this mean that this timeline's Janeway is fated to do the same or can she alter the timeline even more by improving on the previous attempt and possibly making it back alive?
No. The future Janeway altered the past, obviating the need for present Janeway to travel back in time in the future. This evidently resulted in the destruction of the previous timeline (the one that Future Janeway came from) and the creation of a brand new timeline in which the Voyager returns to Earth substantially sooner.
You may wish to note that this is precisely the same plotline as Voy: Timeless, where Harry and Chakotay pinch a Borg time travel device (possibly even the same one used by Janeway in the parallel timeline) and use it to alter the past.
KIM: Wait a second. If I sent a message from the future and changed the past, then that future would no longer exist, right? So, how could I have sent the message in the first place? Am I making any sense?
JANEWAY: My advice in making sense of temporal paradoxes is simple. Don't even try.