1. Open the stack window
2. Load the files needed to edit header.
3. Click the left mouse button to select the first file.
4. Hold Shift and left click on the last file (all files should become highlighted)
5. Right-click any highlighted file and select Change Keyword in Selected Files/Enter Keyword and New Value from the pop-up menu.
6. In the first modal window, enter OBJECT and click Ok.
7. in the second modal window, enter the name of the object and click Ok

This way you can edit any header in FITS-file