How do I underline a letter on my iPhone 10x

Select and edit text on iPhone - Apple Support

After selecting the text you want to revise, you can type, or tap the selection to see editing options:

• Cut: Tap Cut or pinch closed with three fingers two times.
• Copy: Tap Copy or pinch closed with three fingers.
• Paste: Tap Paste or pinch open with three fingers.
• Replace: View suggested replacement text, or have Siri suggest alternative text.
• B/I/U: Format the selected text. (U is for underline)

Bandgeo wrote:

I tried that to underline a password but I don’t have the option of underline

Generally, passwords cannot be formatted (Bold/Italics/Underline) etc.

Also, most apps which allow text input allow formatting ... but there might be some apps which do not permit this!

The article above explains how to edit/format text on an iPhone (iOS 14).