ANSWER: Find and Replace
EXPLANATION: To replace any word with another word, the simplest way is to use the 'Find and Replace' function. Here, the period can be replaced by exclamation point by this function. In MS Word, one has to press Ctrl+H which will pop up the 'Find and Replace' window. In the 'Find What' column, one has to write '.' and in the 'Replace With' column, one has to write '!' and then click 'Replace' button. This will replace the period by an exclamation point withing seconds.