Title tags

I need to change the title tags on my WordPress blog.
This is what I have:
<title><?php bloginfo(‘name’); ?> » <?php bloginfo(‘description’); ?><?php if ( is_single() ) { ?><?php } ?><?php wp_title(); ?></title>

However, when I try to amend it, it goes back to the above.

“You need to make this file writable before you can save your changes” is shown on the page, is this why I cannot amend it?
If so, how do I do this?
I also have a SEO plug in, is this why it won’t take the new code?
Appreciate any help for a very novice WordPress user.
Many thanks