Once you have successfully checked in or approved check in, Check out button will be visible in the Info card of the activity. 
Click on the Check out button and Fill the following list of details in case of late check out: 
Reason for late checkout: Enter the reason for late checkout.
Title: Enter the title for the reason.
Enter a note: Enter the note as a reason for late checkout.
File Name: Upload the file if you want to share any attachments.
Click on the Checkout button.