How to adjust text body width

Let’s see the webpage you’re referring to.

https://www.lindsayulrich.com

Looks like you can do this with CSS.
E.g,

.post .content {
 width: 20em;
}

https://www.w3schools.com/cssref/pr_dim_width.asp

Do you have a section of the dashboard to put CSS, other than in the theme’s files?

ah… i see. And I put this in “Appearance > Editor” ?

Try to avoid editing the theme’s files as when the themes updates those modifications will be erased.

Install this Custom CSS Manager plugin and use its section of the dashboard to hold your CSS modifications.

oh, wow. okay. Thank you so much Andrew!
Very appreciated!

And how would I change it to centre the text?

currently it’s all left-justified