If the version of the entitlement process is currently in use, then you can add new milestones. But, you can’t edit or delete the existing ones. If the versions of the Entitlement processes aren’t currently in use, then you can add new milestones, delete existing one and edit existing ones.

Milestone Edit and Del Action:

SOQL to find Entitlement Process Version dependency:

SELECT Id, SlaProcess.Name, SlaProcess.VersionNumber 
FROM Entitlement 
WHERE SlaProcessId = '<Id of Entitlement Process Version>' 

Milestone Edit and Del Action missing since it is tied to Entitlement Record: