You can just delete the <home>/xml-data/configuration/broker.ks and restart Bamboo that a new key file will be created.
If you're running a Bamboo 6.8+ version, the new certificate will be valid for 20 years. Please see the following BUG report for details:
G’day Community! As we gear up to introduce Bamboo Data Center to the world, we wanted to make sure that we shared a bit more about Bamboo, the product. Our team has put together an overview video ...
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