Blogger World

If your blog has a dynamic template, then use the see-all-posts icon for the blog to get into it the blog-options screen for the blog that you want to edit.

When you are in the see-all-posts mode, the the address bar in your browser is like this:
http://www.blogger.com/blogger.g?blogID=5381944372668369653#allposts/postNum=0

Copy your blogID number - that is, the digits between blogID= and #allposts (it's bold in the example above)

Go go
http://www.blogger.com/html?blogID=NNNNNNNNNNNNNNN
where you have replacd NNN... with your blogID


Using pre-Sept-2011 Blogger, AKA the old  UI / version of Blogger :
Go to the Design > Edit HTML tab

Using post-Sept-2011 Blogger, AKA the new  UI / version of Blogger :
Go to the Design tab
Click Edit HTML
Click Proceed

Note:  the last time I tried this, it automatically opened in the old interface's template editor.   As always, make sure you take a backup copy of the template - see instructions below before changing this.