This will be very easy to do mfp. If you already have a header made for your theme do the following... Open up this directory in your FTP wp-content/themes/mandigo/schemes/teal/images/head.jpg Then once your in this directory rename your new header head.jpg and overwrite. Then reload your blog and you should have a new header.
Ramphix is spot on and this is the easiest way.. also some WP themes have built in theme / header modification under the design tab. Good luck
can i change WP Mandigo theme header height & width, if yes how can i do it? (i think it is possible by modifying style.css.php file)
I am using cpanel. I just create a file similar to the filename of an existing file header. Then I would upload it to the file directory overwriting the existing file. And it's done.