Without changing the markup (like putting a span
around the word "our"), you'll need to use javascript. If you can change the markup, just change it to your desired capitalization.
Here's one way to do it with javascript. Without knowing more about your environment and requirements (like how do you know the word "our" is the one you want to change), I can't say if this is the best solution, but maybe it will get you started.
const h1 = document.querySelectorAll('h1')[0];
h1.innerText = h1.innerText.replace('our','OUR');
<h1> Welcome in our flat. </h1>