To replace a string in your HTML with another use the replace method on innerHTML:

document.body.innerHTML = document.body.innerHTML.replace('hello', 'hi');

Note that this will replace the first instance of hello throughout the body, including any instances in your HTML code (e.g. class names etc..), so use with caution - for better results, try restricting the scope of your replacement by targeting your code using document.getElementById or similar.

To replace all instances of the target string, use a simple regular expression with the global flag:

document.body.innerHTML = document.body.innerHTML.replace(/hello/g, 'hi');