I'm sorry if this is a wrong place. I thought of posting it as this is the category of web design. Actually my problem is with wordpress upgradation.
When I'm trying to upgrade the wordpress from 2.6.5 to 2.7.1, I noticed 2 errors namely:
Fatal error: Call to undefined function wp_remote_request() in /home/onlinedo/public_html/DOMAINNAME/wp-includes/update.php on line 58
and one more error while trying to login is
Fatal error: Call to undefined function wp_remote_post() in /home/onlinedo/public_html/DOMAINNAME/wp-includes/cron.php on line 204
How to tackle it? Any help please.

I would assume you're trying to use the autoupdate feature and it's failing.
First make a backup of your site, including database. Next make another backup and save it on a different computer. Then download wordpress and upload it via FTP. Then visit the admin panel and it will tell you it needs to update the database. Follow the directions and you're done.

I'm not using the auto update feature or plugin. I'm following the usual upgrade prcedure mentioned in wordpress.

Update might not have gone all the way through...back up your stuff and try a completely new install...sucks, but that's the easiest way to go about it, in my opinion.

Try re-uploading all of the core WordPress files. If an error is stating it's making a call to an undefined function you have 2 possible reasons:
1). A plugin you are using is calling to a function that no longer exists.
2). The function isn't defined in the core WordPress system.
If #1, you'll need to contact the plugin author. If #2, then it's possible when you uploaded the new WordPress files, some of them didn't copy over.
Make sure you upload *all* of the files.

Sorry for the late reply! I've solved this problem by using this tutorial.

http://codex.wordpress.org/Upgrading_WordPress_Extended .