Step 1: Go to your Admin dashboard using the credentials provided to you. Usually admin page will be located at - yourwebsitename.com/admin

Step 2: Navigate to Sales > Order from the left menu.

After selecting this, you'll see your orders in the table format in the right side window.

Step 3: Find order you want to change the status and click on view(eye icon) button in the right side, as given below:

Step 4: Select the 'History' tab and change order status clicking the dropdown box next to Order Status field. Order Statuses are pre-defined and you only need to select the required option.

To notify customer, click on the Notify Customer checkbox and you can add any comment you want in the comment box which will be sent to the customer. After that click on 'Add History' button at the right bottom, as in below screenshot:

That's it, you have learned how to change the order status professionally.