How do you change quotations from its original text?

You place brackets around the Words that you have changed in a direct quotation. You should also only do this for grammatical reasons or for clarification... not to mislead the reader or cause the misinterpretation of a quotation.

An example would be Shakespeare's quotation "To be or not to be." If I were worried that young readers might not understand what Hamlet is pondering, I could quote it like this:

"To [exist] or not to [exist]. That is the question."

You would use ellipses if you are only leaving something out, not changing it:

"To be or not to be . . . is the question."