Before installing Coveo for Sitecore, you might wish to review the installation guides.

Coveo for Sitecore Installation Guide: 
https://docs.coveo.com/en/2219

Upgrading from Coveo for Sitecore 4 to Coveo for Sitecore 5:
https://docs.coveo.com/en/2642

Best Practices When Upgrading Coveo for Sitecore: 
https://docs.coveo.com/en/2155