Hi @Josh4u
You can change this using CSS transformations.
If you edit your invoice template by clicking on the green box in the top left:
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Select ‘Customise this Style’:
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Then, at the very bottom of the CSS code, add these lines:
.transform-taskHead_02 {
content: 'Description';
}
Like so:
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Press save, read and acknowledge the message, and job done 
For reference, there’s details on this feature here: