a. An amendment can formally be proposed by congress to the constitution by a 2/3 vote in both the house of representatives and the senate.
An amendment can formally be ratified by state legislatures by a 3/4 vote of the states.

b. One informal way of changing the constitution is Judicial Review, where the supreme court can claim acts of the other branches of government unconstitutional. It can also declare certain laws applicable even if they are not specifically stated in the constitution. An example of this is the court case Marbury v. Madison. Another informal way the constitution has changed is through technology. An example of technology shaping the constitution is it gives the president an increase in power in the role of commander in chief with the creation of atomic weapons and new electronic communications.

c. Informal methods are used more often because their easier and much faster than formal amendment processes. It takes a significant amount of time to have a constitution voted on and ratified and usually the government doesn't have the time to go through this long procedure.