How do I change my "target" calories for each day?

I think it's a premium feature, but you can hack together a manual version by Quick Adding to reduce your remaining calories and creating a custom cardio exercise to increase them. I have one called "Banked Calories."

The way it works in practice is that each week, I prelog all of my meals and snacks Monday through Friday, then Quick Add my remaining calories rounded down to the nearest 100 so the diary doesn't show a huge amount remaining. I total up the number of calories I Quick Added each day M-F, then log that many Banked Calories under Exercise on the weekend so I have a little more room to play on Saturday and Sunday. Or other days of the week, like if I know I'm going to have something heavier for dinner one day or if my workout is going to be more intense I might allocate a few hundred leftover calories to that day. This week, for instance, I've Quick Added about 900 calories across 5 days, so I have an extra 450 each day to play with on Saturday and Sunday.

Not premium if you just want to set a static goal.

But it sounds like you want to vary, so ditto's to tip above - either a self made private food items for a number of calories, or a workout item of set calories.