Well, both are correct and you can use either. They just have slightly different nuances. "I have read and understand" = I have read + I understand (now, as a result of reading it). "I have read and understood" = I have read + I have understood. That is, you understood it when you read it.

I have read and understood the ... , OR I have read the ... and I understand it.

I have read and understood the....

I have read and understand is correct. Understood would be past tense and you want to say that I understand it now.