like you upgrade to wp 2.7, backup your current wp folder. download wp 2.6 files and replace it with current wp 2.7 files keeping wp-config file & wp-content folder as it is.
once you replace all files try to login to your wp dashboard an you will see message for wordpress database upgrade. follow that link and we have done it you should see your blog back to wp 2.6.
please keep in mind that its not suggest to downgrade wp version you may lose your blog data or end up with buggy wp.choice is yours.
Always backup database and core files before proceeding for upgrade/downgrade.