How to upgrade Cloudera manager 6.3.0 express version to cloudera manager 7.3.1 express version
The short answer is: you don't. There is no "cloudera manager 7.3.1 express version" available.
Cloudera Express was discontinued over a year ago now, please see the announcement here: Cloudera Enterprise 6.3.3 Released
…scroll down to the subsection Important Note About Cloudera Express