Learn Spanish (Latin America): Quiero que cambies esa actitud.

Because the sentence starts with Quiero que (I want that). When you express a desire or expectation for someone else to do something, Spanish requires a verb form called the subjunctive. For cambiar (an -ar verb), the tú (you) form in the present subjunctive swaps the usual -as ending for -es, making it cambies.

Why is there a que here when the English translation is "I want you to change"?

In Spanish, you can't literally translate "I want you to do something." Instead, you have to use a structure that literally translates to "I want that you do something." This is why we need que (that) to connect Quiero (I want) with the action you want the other person to do (cambies).

What would it mean if I just said Quiero cambiar esa actitud?

That would mean "I want to change that attitude" (meaning you want to change your own attitude). When the person wanting the action and the person doing the action are the same, you just use the infinitive verb (cambiar) without que. To show that you want someone else to do it, you must use que and conjugate the second verb.

Why is it esa and not ese?