To change the Subject Line of a Reply in Gmail, just follow the steps given below:-

Step 1. For changing the Subject Line of a Reply in Gmail open the email for which you wish to change the subject line.

Step 2. Click on the drop-down arrow.

How to Change the Subject Line of a Reply in Gmail

Step 3. Choose ‘Edit subject’¬†and for Gmail subject line change the subject will appear auto-selected.

How to Change the Subject Line of a Reply in Gmail

Step 4. The window will pop out enabling you for Gmail subject line change.

How to Change the Subject Line of a Reply in Gmail

From here you can easily change the Subject line in reply of any Gmail. If you have any doubt, ask in comments. Thanks.