Hi i want to update mac 10.6.8 to 10.7.4 how can I do it?

First check that your Mac meets the System requirements for Lion...

Lion Tech Specs > http://support.apple.com/kb/SP629

If so Contact Apple in your Country to Purchase a Lion Download code...

http://support.apple.com/kb/HE57

In the US...

1-800-MY-APPLE or 1-800-676-2775

Be sure to make a Backup of your Current System Before Upgrading...

Also... Check here for compatibility of 3rd party Software you may be using...

http://roaringapps.com/apps:table

You should also consider more RAM... Lion is a hungry beast...

It is important to get the Correct and Matching RAM

See Here > OWC RAM > http://www.macsales.com