There are several pretty good guides. 6 to 7 is a bit tricky. 7 to 7.5 is a bit of work too and anything after that is very smooth.

6 to 7 check out 

http://joelabrahamsson.com/upgrading-a-site-from-episerver-cms-6-to-episerver-7/

If you are using composer for 6 then you need to convert this to new blocks functionality. 

7 to 7.5 check out 

http://world.episerver.com/documentation/Items/Installation-Instructions/Installing-EPiServer-updates/Upgrading-to-EPiServer-75-CMS/

Main issue here is to convert files to new media files that 7.5 uses. There is a tool to convert all old files to new media content.

After 7.5 it's a piece of cake to upgrade with nuget packages from visual Studio. Might as well go to latest version 9 (at the time of writing)

Hi Daniel,

Thank you for reply.For composer migration I am checking EPiServer.ComposerMigration.

http://world.episerver.com/documentation/Items/Installation-Instructions/EPiServer-CMS/Version-7/Migrating-Composer-websites-to-CMS-7/

Looks promising for the content migration to blocks yes. Didn't know about that tool :)

Remember to backup both database and files after each step. You will likely run the upgrade quite a few times :)

Having a fresh alloy demo site of each version with correct config everywhere is also good. 

Yes Daniel will take care of that. Thanx for help...!!