As stated in :h title
:
When on, the title of the window will be set to the value of
'titlestring' (if it is not empty), or to:
filename [+=-] (path) - VIM
Where:
filename the name of the file being edited
- indicates the file cannot be modified, 'ma' off
+ indicates the file was modified
= indicates the file is read-only
=+ indicates the file is read-only and modified
(path) is the path of the file being edited
- VIM the server name |v:servername| or "VIM"
So you'll want to modify the content of titlestring
.
See :h 'titlestring'
for examples and explanations.