Of course, you need to go workflow for this issue type, then found transition "Decline", and rename it.
Info about transitions and workflows terminology here
I don't think you can actually change the name of these buttons and nor did I find any feature requests for them. I looked at the properties and options on our approvals and did not see any text. Even looked in the xml of the workflow.
Perhaps you should raise a feature request with Atlassian for this improvement.
Hope that helps
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