Do I stick to base calories intake or up it if I walk 10000 steps or exercise 45mins?

If you have a good brand/model fitness tracker synced to MFP - as it sounds like you're describing - you eat those extra calories, too.

It's OK to save some of those calories and eat them later, using your weekly average calories in MFP as a guide.

That's the general rule.

As long as you're not already trying to lose super-fast, and it's a rare day, and you're not actually hungry, it's OK to let that extra activity speed up your weight loss.

But under-eating too often tends to backfire. After a while, it just gets too hard, and we tend to over-eat in compensation, or give up altogether. If we somehow white-knuckle through ultra-low calories for a longer time, health risks increase, and we may see things down the road like hair thinning, fatigue, weakness, or muscle loss. No one needs that. Faster weight loss isn't necessarily better weight loss.

To explain the concept, I'll use an example:

Let's say Susie could eat 2000 calories daily to stay at her current weight.

Susie joins MFP, tells MFP she wants to lose a pound per week. MFP will give her a calorie goal of 1500 calories per day. That's because a 500 calorie daily deficit results in about a pound per week of weight loss.

One day, Susie gets 400 calories of exercise on top of her usual activity. That means that on that day, she burned 2400 calories, rather than 2000. Therefore, knowing that she wants to lose a pound a week, MFP tells her she can eat 1900 calories instead of 1500.

Why? Her usual 2000 calories minus the 500 calorie deficit is 1500 calories. Her unusual 2400 calories minus the 500 calorie deficit is 1900. Eating the number of calories indicated would be expected to result in Susie losing about a pound a week. She eats different calories on different days depending on how active she is, but keeps the same calorie deficit. It's the calorie deficit that triggers weight loss.

All of that assumes that Susie is very average in her calorie needs: That's how MFP and a fitness tracker produce estimates. They use research findings about average people.

But not everyone is average in their calorie needs. Most people are close, but a few can be farther off. What if Susie isn't average?

After 4-6 weeks (or one menstrual cycle if she has those), if Susie stuck pretty close to the calorie goal MFP gave her, she can compare her actual average weekly weight loss to the one pound per week rate she asked for. If it's pretty close, she's all set going forward to stick with that. If it's not close, and she can make a sensible, sustainable adjustment, she can use her actual experience to personalize her calorie goal and make weight loss more predictable going forward.

One thing to add: If you have a tracker synced, I'd suggest you turn on "negative adjustments" in MFP. That way, if you have a rare day where you're much less active than usual, MFP will subtract calories from your goal to keep your deficit the same as on other days. If that's not turned on, MFP will only add calories. If your activity is rarely under the number of calories MFP expects, maybe that's OK . . . but if it's frequent, weight loss would be slower than your goal.

Best wishes!