According to these release notes Intellij 2017.3 and later should have a feature that enables easy migration from JUnit 4 to JUnit 5 (a.k.a. JUnit Jupiter).
However, that's not appearing for me. How do I get it to show up?
If it's not showing up, then the inspection is turned off. You can enable it in the Settings by going to:
Editor → Inspections → Java → JUnit → JUnit 4 test can be JUnit 5
Also, make sure that JUnit Jupiter is on your classpath.