Are you planning an in-place upgrade or will the new OS be on different hardware?
What is the current architecture (standalone Splunk, indexer cluster, etc.)?
Start by reading the Release Notes for all of the versions since 7.3.4.2. They will explain any steps you must take to upgrade to (or beyond) that version.
Install and run the Splunk Upgrade Readiness app to identify any Python 3 incompatibilities in your environment.