In a rpg one of the best things for players is mystery.
"What will await behind that door?"
"How can I defeat this monster?"
"How much does it take to kill it?"
"Will I use my most powerful spell or will I save it for later?"
Mystery brings choices. And a rpg (D&D) is fun when players have to choose.
It ain't easy to have mystery: players are veteran (or they become so after a few), know very well manuals, and access to a lot of informations through the web.
It's not a bad thing; it means players are involved in the game and keep themselves adjournated.
After a few sessions eventually your players will say something like: "Ah!Gnoll!! They've got an average of 20 hp and AC 15!!"
So it's a good and just thing (and a right of yours as DMs) to modify the base stats of monsters.
Change hit dice, AC, attack modifier (or Thac0 if you're still playing older editions) and damage. Obviously modify also xp and CR (last one only for edition 3.0 or higher).
Do it every time you feel it deserves so or according to situation.
You can do it using only your experience or you can use the following random table (most old school gamer will remember a similar table in the first edition setting of Forgotten Realms) rolling a d100:
01-10 monster (or NPC) has one hd or level lower than normal
11-60 monster (or NPC) has normal hd or level.
61-75 monster (or NPC) has one hd or level higher than normal
76-90 monster (or NPC) has 2 hd or level higher than normal
91-100 monster (or NPC) has 3 hd or level higher than normal
You can also give monsters special attacks, defenses or immunities.
The most you modify the most your players will be shrouded in mystery and the most they will have fun.