Hit U, go to the "Assignments" tab and you will see the officer icons on the right side. Double click on the officer you ant. He will ask you if you want him to join your crew or train a member of your crew.
Training is how you replace the various skills your current bridge officers have.
For example, say you have a Engineering Officer whose first skill for Space is Emergency Power To Engines, and you want Emergency Power To Shields.
So you get an Engineering Officer candidate who has Emergency Power To Shields. Double click and hit 'Train'. A list of your current bridge officers appears. Say Ensign Jones is your Engineer. You select Jones, and the Candidate will tell you he will replace your BO's current skill of Emer. Power to Engines with Emer. Power to Shields.
If that is what you want, click on Train.
If you were to accidentally select your Tactical Officer by mistake, the Officer Candidate will tell you he can't train that BO.