How To Iterate Dictionary Object

Dictionary is a object that can store values in Key-Value pair. its just like a list, the only difference is:
List can be iterate using index(0-n) but not the Dictionary. Generally when we try to iterate the dictionary we get below error:

"Collection was modified; enumeration operation may not execute."

So How to parse a dictionary and modify its values??
To iterate dictionary we must loop through it's keys or key - value pair.

Using keys