If time were linear there would be a beginning and an end. A line always has an end, no matter how long that line is. Time never began and time will never end. Therefor time cannot be linear. Time is a loop. What is the only shape that you can keep going straight only to end up back where you began. A Circle or sphere. The earth is a sphere and if we took a nonstop flight across the world, we would end up right back where we started by going in nothing but a straight line. Time from our tiny distance within it appears to be a straight line. We just haven't looked far enough into the future to end up looping back around to the past which brings us to the eternal present or now.
Since time is a loop we have a linear future, a linear past, and a fixed present. When we embrace the entirety of our time our past, present, and future all merge into one continuous time loop. The past is not separate from the future in the same way two people who live on separate sides of the globe aren't separate from the world.
Since our time is a loop where past, present, and future are not separate but one we have the ability to change any part of our time. We can change our past in the past, present, or the future. We can change our future in the past, present, or future. We can change our present in the past, present, or future. Time is not an endless fixed line where you just keep moving forward and can never go back. Time is a loop where you can go back by going forward. You can go forward by going back. Or you can live in the eternal present. Love and Peace!