How can I read or use safari in landscape mode

Swipe up from the bottom of the screen to bring up the Control Center. On the left most pane, look at the icon at the right, the one with a circular arrow with a padlock in the center. I suspect you're going to see that it's red. Tap it to turn off rotation lock.

https://support.apple.com/en-us/HT204547

Open control center and find the button for orientation lock and turn it off so that you can view certain apps in landscape mode.

I'm more of a visual person so my next door neighbor said, go to Utube website and write in on my iPhone 6+ landscape mode. There were a couple videos from different people and just watch all of it.