How to modify text width of a page?

As you can see,

Pleae post a link to your site.

This is my website: https://www.haichaoyu.com/

Change your CSS from

.entry-content, .singular footer.entry-meta, .singular #comments-title {
    margin: 0 auto;
    width: 68.9%;
}

to

.entry-content, .singular footer.entry-meta, .singular #comments-title {
    margin: 0 auto;
    width: 100%;
}

@sterndata

Your solution works perfectly! Thanks very much.

use

.content {
  width:100%;
}

To add CSS: If you are using WordPress 4.7, use the “Custom CSS” option in the customizer. If your theme has a custom CSS option, use that to add the CSS shown above. If not, install the plugin Simple Custom CSS. Or, if you have Jetpack installed, enable its Custom CSS module.

Learn the Chrome Developer Tools to help you see and test changes to your CSS.

satyendra: If you have other questions, please start a new thread rather than jumping in to an old one. thanks.