How do you express that your understanding of something is completely changed?

You can call that an epiphany:

• a moment when you suddenly feel that you understand, or suddenly become conscious of, something that is very important to you

(Cambridge Dictionary)

• an illuminating discovery, realization, or disclosure. A revealing scene or moment.

(M-W)

As tripleee suggests, paradigm shift is another term that applies to the concept:

an important change that happens when the usual way of thinking about or doing something is replaced by a new and different way: This discovery will bring about a paradigm shift in our understanding of evolution.

(M-W)