Select the My Site icon in the bottom navigation bar and click the arrow beside your site address.

In the Android app – For WordPress.com sites, tap the three dots on the upper right of the screen, then tap Show/hide sites. Tap a site you would like to show or hide, then select the option to hide or show (whichever is applicable) at the top of the page.
In the iOS app – Swipe from right to left on the site you wish to hide, then tap Hide.