Take a copy of it.
I'm sure there's more to this question, but you haven't really told us what your question is. To backup data, you take a copy of it and put it somewhere else. That's it. So you need to expand the question.
There is one complete description here: https://confluence.atlassian.com/display/DOC/Migrating+Confluence+Between+Servers
But as Nic points out it is hard to give detailed advice when the question is vague.
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