Hi Arun,
The easiest way is to update your whole Odoo environment at once. Open up a terminal and navigate to your Odoo server:
cd /odoo/odoo-server/
Fetch the new code from Github, notice that 9.0 is for V9, 8.0 for V8 etc.
sudo git fetch origin 9.0
Now apply these updates to your Odoo. Do notice that a git reset --hard will overwrite all files, so only use this is you didn't add code in default Odoo modules (which is against the rules anyways):
sudo git reset --hard origin/9.0
The new code has now been added to your Odoo, now restart your Odoo service and update all your modules to reload all the new code in your Odoo. Something along the lines of:
./odoo.py --xmlrpc-port=8080 -u all
You now have a fully updated Odoo.
Yenthe