XF 1.5 Upgrading to New XenForo

Alright so, I have a XenForo forum that I bought in 2017. I haven't done much work on it but I am planning on cleaning it up and changing the theme to a different theme. The Xenforo is the 2017 version. How can I upgrade it to a new Xenforo without all my links breaking? Any suggestions on this?
 
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