Solution:

We shall meet in room number four hundred and twenty. Beware! We are being followed. The new password is: DETOUR. See you at four thirty. The puzzle was a cryptogram with a clue "U equals A". So, every letter instance of U needs to be replaced with A. After that, the entire solution is based on one's intuition via the trail and error method. Here are a few tips that one can use to solve a cryptogram. 1. One can look for one-letter words. The single letter is, more often than not, 'a' or 'I'. The exact letter can be determined by examining the use of the letter in other words. If a three-letter word begins with that same letter, it is almost certain that the letter is "a". There are more common three-letter words beginning with "a" than with "I". 2. Try to identify the vowels. If you can find five letters with these properties, you have very likely identified the vowels. Vowels account for almost 40% of the letters in English text and are almost present in every word. The most common vowel is "e"; the least common is "u". 3. The word "the" is extremely common and can be identified by finding words like "that", "there", "then" or "than". 4. If you get "t," "h," "n," "e," and "a" solved, you're almost through. 5. A single letter after an apostrophe is usually either "t", "s", or more rarely "d". Two letters after an apostrophe usually indicate "re" if the letters are different or "ll" if both letters are the same. 6. Conjunctions like "but" or "and" often follow commas. A question mark often implies a "wh" in the clause preceding it. 7. Look for pairs of two-letter words, one beginning and the other ending with the same letter. That letter has a good chance of being "n," "o," "s," or "t," and the second letter of the word which starts with the shared letter is likely to be "f," "n," "o," "r," "s," or "t." 8. If you find two two-letter words where the letters are reversed, you've got either "no" and "on." You just have to figure out which is which!