There are a few ways to Break treaties or agreements in Supreme Ruler 2020.
1. The easiest, and the one that appears to have the least negative consequences, depending on which treaties are active, is by pressing the Break All Ties with Region button on the trades screen, as seen in the screenshot of this article. A small confirm dialog opens at the cursor location, you need to press this to finalize your actions.
2. Another way is to go to war, but this might not always be desired.
3. The hardest way is to get the AI to accept the cancellation of a treaty. This is done from the diplomatic exchange dialog.
Beware that breaking treaties will usually have some negative diplomatic consequences. I have never actually seen the AI accept the proposal to break a treaty, but if you manage to do it, this may actually be the least hurtful way to break a treaty.
Nothing happens, a rare bug?
Sometimes you may experience that nothing happens when pressing the break all ties button, a simple save and restart should fix this problem. When you then press the button after re-loading your game, you should see a small popup confirmation box at your cursors location, click this to break ties with the region.