How do I turn sound higher on a Apple SE phone

Use the volume buttons on the left side of the phone. Note that the buttons control the ringer volume when the Home screen is shown, the same buttons control the sound volume when on a call, and the same buttons control the volume level for apps when pressed while in any other app.

You can also control volume with the volume slider in the Control Center and the slider in Settings/Sounds & Haptics for ringer and alerts only.

If the volume is low during a call, you can use the volume buttons on the side of your iPhone.

To turn up the ringtone, go to:

Settings -> Sounds & Haptics. Turn up the ringtone on the scroller