Time Travel is a surprisingly widespread phenomenon. It has been observed countless times throughout The Multiverse in many different ways.
Actually changing the past, or having any real effect on your timeline is, in fact, near impossible. This is due to the unspoken Rules of Time, where every time you try to interact with your own timeline, it instead creates a parallel universe wherein that change applies. That means your original timeline won't be affected, negating any potential paradoxes.
Time Travel Methods
The Time Watch and The Time Gem are the only known items in The Multiverse capable of directly meddling with the past without creating new universes. The most common way of achieving the same goal would be through Multiversal Dissonance, as seen with: The Gascoin Race, The Paladin and Darkside.
The Master of Darkness was capable of creating Time Circles, which allowed for the users consciousness to be transferred to a similar, yet parallel universe counterpart of them, at any point in history after their birth, and before the transfer.
417 was able to create parallel universes by sending his memories back in time, to be received by an earlier version of himself; an ability he referred to as Time Leap.
Running quickly enough could open a wormhole which would allow you to travel in time. However, this was extremely dangerous and could in some cases lead to spontaneous combustion or loss of abilities.
Noteworthy Time Travelers
Shinchi Takahashi had the ability to Time Leap, though it wasn't originally his.
The Streak created multiple universes as a result of his travels.
Echo Chara was seemingly able to navigate the timelines freely and could posses anyone she deemed fit as a host body.