You can adjust the price of your app at any time through your iTunes Connect and Google Developer accounts. Setting a price for your app means that users will need to pay to download your app from the respective app store. Keep in mind, Apple and Google both take 30% of the revenue from app sales.


See here for Apple Developer's full instructions.

1. Log into iTunes Connect.
2. Click My Apps.
3. Select an app.
4. On the left menu, click Pricing and Availability.
5. Select a price from the drop-down menu.
6. To make this change immediately, click Save. To schedule this price change on a future date, click Plan a Price Change and select your dates, then click Save.


See here for Google Developer's full instructions.
Note: If an app is published as Free, you cannot change it to Paid later.

1. Sign into your Google Play Developer Console.
2. Click All applications.
3. Select an app.
4. On the left menu, click Pricing & Distribution. (Before setting a price, you can review Android Developer's list of currencies and price ranges allowed by country.)
5. Under "Pricing & Distribution," type a price.
6. Near the top of your screen, click Save draft (new apps) or Submit update (existing apps).