How do you have "the discussion" with your spouse that he/she needs to improve in bed?

Depends on your relationship, your style of interaction with your spouse. First off, there will always be things you cannot change about the other person. That said, you could always ask "do you like X, Y or Z?" or say something like "I really enjoy X, Y or Z." Get the conversation going. If there's something you'd like to try, suggest it: "what do you think of doing X, Y or Z?" If you and your spouse are comfortable talking about it, you don't have to tip-toe around the topic - just ask.