Which version of FlymeOS (based on Android) are you running?
Have you recently downloaded any new apps, or has the phone updated the OS?
The most common way to uninstall any Android app is via the Settings > Apps > 'Installed' list (
see video) > Scroll down to the app you want to remove > Uninstall.
Alternatively, go to Google Play Store, search for Yandex or from the My apps in the sidebar, find Yandex under Installed and Uninstall it.