Select the My Sites (W) icon in the bottom navigation bar and select Switch Sites.
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.
For self-hosted sites, select the site long press on it (hold your finger down for a second or two) then a pop-up will appear asking you if you want to remove the site from the app. Tap "yes."
In the iOS app - Go to My Sites then swipe from right to left on the site you wish to hide or delete then tap "Delete" or "Hide" as applicable.