SugarCRM can not be upgraded directly from version 5.2.x to 6.2.x. Its a multistep process. You have to upgrade SugarCRM to version 5.5.zx then from 5.5.x to 6.2.x. Below is the outlined procedure for that.
Step1: Check the server for Apache,PHP and MySQL version. (My server was having lower versions so I had to upgrade php and mysql before upgrading SugarCRM)
Step2: Login to SugarCRM with admin user or same access level user.
Step3: Click on Admin link
Step4: Scroll down and Click on Upgrade Wizard
Step5: Click Next, Next, when it asked for upload upgrade file go to http://www.sugarforge.org/frs/?group_id=6 website and download the suitable file according to your current version. I have download all the below files once as to avoid download them again and again.
Step6: After upload the suitable file complete the steps and on last step it will give you link of “DONE”. Click on that and logout
Step7: Repeat the step4 to step6 again for upgrade to 5.5.1 to 6.0.4 and one more time for 6.0.4 to 6.2.0