The Quick Editor is available from views with a track list and can be toggled on/off from View > Quick editor.

When you select one file with the Quick Editor visible, the fields will be populated with the tag data from the selected file.

You can directly change the contents and then press Save changes to apply them to the file.

When selecting multiple files, this editor will work in a little special way:

-For fields that contains the same value for all selected files (e.g. album, year, genre) this specific value will be shown.

-If a field (e.g. title) differs between the files, the field will show -Keep original value-. Unless you change the value of the field, each file will keep it's current value for this specific file when saving changes.

The editable fields displayed in the Quick Editor can be changed by clicking the Pen icon in the bottom of the Quick Editor.

When clicking that icon a new window will be shown, displaying the selected and available fields.