In LiveAgent, there is an option to automatically change the ticket subject on every customer's received reply. This option is useful in cases where customers send you an initial email with a general subject like "question" or with the subject completely empty (in such cases, the ticket subject will match the customer's message) and update the subject later on, or if they reply to the same email thread but regarding a different topic.
Activate the setting
To activate the Automatic ticket subject update function, navigate to Configuration > Email > Email settings, and check the option "Ticket subject update - Update ticket subject from every received reply".
How it works
The subject of a ticket that is populated to your replies is always noted in the ticket's tab name, in the header above the department name and tags, and in the Subject field in the ticket's details section.
Note that the final form of the subject in your message depends also on the template setup in Configuration > Email > Customer templates.
With the "Update ticket subject from every received reply" setting active, this subject will be automatically updated on each customer's reply.
How to update ticket's subject as an agent
If you need to change the subject as an agent, changing it in the agent's reply will change it just for that reply. If you need to keep the updated subject also in the following agent's replies, change the subject in the Ticket details section in the bottom right section instead. It still applies that if the customer sends you another reply with a changed subject, the subject changed by the agent will be overwritten, as explained above.