Upgrade from ExpressionEngine 1.7.0

Hi everyone, hope someone can help me or at least give me some pointers on what to do here… I’m the new admin for a website using EE 1.7.0; the page has been around for a while, nobody knows who configured it or give support before so I’m on my own. Now if I want to update where should I start? …not sure what to do here because even the link to “user guide” on the version I work on leads to a 404 error. Any suggestions will be much appreciated. Thanks in advance 😉