Target allows you to edit a live activity. Be aware that editing an activity that is in progress could reset the test, so reports might not recognize some of the changes.
It is safe to make small changes, such as editing existing text or html offers.
The specific actions that reset experience names and reports are:
Adding a new location
Deleting a location
Adding new offers or deleting offers from an exiting location
Once you have added a new offer or swapped the existing offer with the new one it will be disabled. But these changes will affect the reports and experience names as stated above.
See the entire troubleshooting doc if you have more questions.