Changing existing installation to default to WWW instead of non www

First, test if both work now. They should if host has setup www. correct for you.

May I ask why you need to do this?

What many do, in order to keep SEO, is redirect either www. to non-www. or vs. versa.

https://www.seomoz.org/learn-seo/redirection

It’s a legacy site that I’m looking at. There are some inconsistencies with the setup I’m trying to sort out.

At the moment both do work, but it’s not redirecting the way I would expect. e.g. If I bring up https://www.my_website.com it goes to https://www.my_website.com. However all the other pages redirect to non www prefixed pages. e.g. https://www.my_website.com/page1 would redirect to my_website.com/page1

A) Add the WWW. in the URL’s in settings.
B) Once set, resave permalink setting.
C) Add this to top of .htaccess above wp rewrite rules, not in them, not below them, above them:

# BEGIN WWW rewrite
<IfModule mod_rewrite.c>
RewriteCond %{HTTP_HOST} *!^www*.seomoz\.org [NC]
RewriteRule (.*) https://www.seomoz.org/$1 [L,R=301]
</IfModule>
# END WWW rewrite

Use in place of seomoz your domain
Use in place of .org your tld (.com usually)

Really appreciate that thanks.