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Tap the App Store app icon, which resembles a white "A" on a light-blue background. It's in the lower-right side of the App Store screen. This is at the top of the screen. Type in adblock plus, then tap Search in the keyboard. It's to the right of the Adblock Plus app icon, which resembles a stop sign with "ABP" written on it. Press the Home button, then tap the Settings app icon, which resembles a grey box with gears on it. On an iPhone X, you'll swipe up from the bottom of the screen to minimize the App Store. It's near the middle of the Settings page. This option is near the bottom of the Safari page. The switch will turn green {"smallUrl":"https:\/\/www.wikihow.com\/images\/1\/15\/Iphoneswitchonicon1.png","bigUrl":"\/images\/thumb\/1\/15\/Iphoneswitchonicon1.png\/46px-Iphoneswitchonicon1.png","smallWidth":460,"smallHeight":300,"bigWidth":"46","bigHeight":"30","licensing":"<div class=\"mw-parser-output\"><p>I edited this screenshot of an iPhone\n<\/p><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"http:\/\/en.wikipedia.org\/wiki\/Fair_use\">Fair Use<\/a><br>\n<\/p><\/div>"}. Open your iPhone's Safari browser and go to https://www.youtube.com/ to access YouTube's mobile site. Due to the Adblock Plus extension, you'll be able to watch ad-free YouTube videos here.

Summary:
Open your iPhone's  App Store. Tap Search. Tap the search bar. Search for Adblock Plus. Tap GET. Open your iPhone's  Settings. Scroll down and tap Safari. Scroll down and tap Content Blockers. Tap the white "Adblock Plus" switch . Watch YouTube videos without ads.