Please, paste this code in wp-content/themes/twentyeleven/style.css:
For site title: After line 518 :-
font:30px "Bookman Old Style" !important;
For site description: After line 529 :-
font:14px "Bookman Old Style" !important;
Don’t forget to update your required font name 😉
mr49online's Blog 
Comments on: "Twenty Eleven: Chainging header font." (10)
Thanks again for all of your help with my site tweaks. I really appreciate it! You are a great ambassador for the WordPress.org community!
Its my pleasure…. :). Thanks for the complement.
Laura Zera! this post is dedicated to you. 😉
Hey admin, extremely informative blog post! Pleasee continue this awesome work..
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Hi
I’m literally in the process of making my first ever blog and although WordPress is superb it is daunting for me at least. I am experimenting with setting up the blog using Twenty-eleven and for some reason I can change the header font size but not the type. I’ve added the line as per your recommendation but it makes no difference. What am I doing wrong?
Hello,
double check the line you added in your style.css file. I am sure nothing is wrong in above post, as you can read others comments, it works for them 🙂