How can I make my app to be free for certain period of time? Is it ok to switch it to free and then after a day or two switch back to my original price, is it possible and where can I do it?
Yes you can change from free to paid and vice versa. More conveniently you can schedule prices changes.
Scheduling Price Tier Changes
If you have a Paid Applications contract in effect, iTunes Connect allows you to schedule price tier changes for your apps over time. Scheduling is useful for sales and other temporary pricing changes that have a definite beginning and end date, as well as permanent pricing changes that have no end. For example, you might offer a promotional price for a month and then return to the regular price.
To schedule price tier changes, you must have already submitted the app; the options for price scheduling appear only when you are editing an already submitted app. For each price tier, you can set an effective date (the date when the new tier will take effect on the App Store), and an end date (the date when the tier will no longer be in effect and will return to the previously set price tier).
You can set up as many price tier changes as you want in advance and your app will change prices on your effective dates, but no interval can begin more than a year in the future. Your price tier schedule will show at all times on the “Rights and Pricing” page for your app. There you can track when your price changes will occur.
An app can be paid then switched to free but an app can never be switched from free to paid. You would need to release a new app if you want your free app to be paid. The app would then be subject to apple's review process again.
Paid -> Free = Okay
Free -> Paid = Not Possible