There’re two ways to update Paper Plane: automatic and manual.
Please go to Appearance → Theme Dashboard and make sure that you installed the recommended plugins.
In the “Next Steps” section click the “Enable automatic theme updates” link and follow the instructions to enable automatic theme updates.
First download the latest version of the theme by going to your ThemeForest account, the Downloads section. Locate the theme, click on Download button and select the “Installable WordPress file only” option.
Navigate to Appearance → Themes and click the Add New button. Then click Upload Theme.
Select the downloaded theme archive on your hard drive and click Install Now.