Javascript replace text string with h3

Hi,

Using Chrome Dev Tools I have found the CSS selector of a H3 heading.

#container > div.my-table > h3

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Within the H3 heading is "Test - some more text here". I need to replace "Test" with "New".

The following attempt results in an 'undefined' error message -

document.body.innerHTML = document.querySelector('#container>div.my-table>h3').replace("Test", "New");

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Can anyone see what I am doing wrong?