Need to change innerHTML of H1 tag on multiple pages

I'm trying to change the innerHTML of H1 tags on multiple pages on my site using GTM. The site I'm working on currently takes the H1 from the URL slug and I am unable to edit the H1 directly in the CMS. I figured If statements would be the way forward, changing multiple H1 tags across different pages by stating the current H1 tag and if it matches, changing it to what I want it to display. I've been testing in the console.

Here what I have so far:

document.getElementsByTagName('h1')[0].innerHTML
if ('h1')[0].innerHTML = "This" .innerHTML ="That";

This works correctly in the console, but doesn't change anything on the page so clearly it's not working .

I am not at all experienced with Javascript and am not particularly tech-minded so please bear this in mind with any answers :)

document.getElementsByTagName('h1')[0].innerHTML
if ('h1')[0].innerHTML = "This" .innerHTML ="That";