How to change "permalink" structure in "Wordpress Thesis" theme?

You can do it using .htaccess but I would take a look at WP Rewrite Class , it may also be what you're looking for for this kind of thing.

http://codex.wordpress.org/Class_Reference/WP_Rewrite

As the rewrite rules are a crucial part of your weblog functionality, WordPress allows plugins to hook into the generation process at several points. rewrite_rules(), specifically, contains nine filters and one hook for really precise control over the rewrite rules process. Here's what you can filter in rewrite_rules(): To filter the rewrite rules generated for permalink URLs, use post_rewrite_rules. To filter the rewrite rules generated for dated archive URLs, use date_rewrite_rules. To filter the rewrite rules generated for category archive URLs, use category_rewrite_rules. To filter the rewrite rules generated for search URLs, use search_rewrite_rules. To filter the rewrite rules generated for the latest comment feed URLs, use comments_rewrite_rules. To filter the rewrite rules generated for author archive URLs, use author_rewrite_rules. To filter the rewrite rules generated for your Pages, use page_rewrite_rules. To filter the rewrite rules generated for the root of your weblog, use root_rewrite_rules. To filter the whole lot, use rewrite_rules_array. The action hook generate_rewrite_rules runs after all the rules have been created. If your function takes a parameter, it will be passed a reference to the entire $wp_rewrite object.

Hope this helps..

EDIT: Added this link in here as it may help you out as well with what you're trying to accomplish. http://www.dev4press.com/2012/tutorials/wordpress/practical/how-wordpress-url-rewriting-works/

first set it to default url & see whether eveything is right, then delete all text in .htaccess & add the new url format you need & save & get .htaccess updated & see your site