Assuming you have access to the knowledge base, there are several good articles on this:
https://support.ptc.com/appserver/cs/view/solution.jsp?n=CS100514
https://support.ptc.com/appserver/cs/view/solution.jsp?n=CS155757
https://support.ptc.com/appserver/cs/view/solution.jsp?n=CS189560
https://support.ptc.com/appserver/cs/view/solution.jsp?n=CS18485
Basically, look at these two preferences:
Allow checkout non-latest iterations:
Site > Utilities > Preference Manager > Create & Edit > Allow checkout of non latest iterations = Allow checkout of non-latest iterations
Allow revise non-latest revisions:
Site > Utilities > Preference Manager > Revise > Allow revise of non-latest revisions = Yes