There are two options for you. The first is to upgrade the module by command, the second is by interface.
The thing is that whenever you change XML/Python files you will need to update your module as Odoo does not automaticly update the code at this point. (this is an idea that will probably come in Odoo 9 though)
Option 1 - by interface:
1. Go to settings and update your module list by clicking on 'Update module list'.
2. Search the module you've updated in your list, open it and click on update.
3. You should now see your customizations. However this does seem to fail once in a while for me, this is why I like option two.
Option 2 - by command:
1. Open up a terminal and type in the command ./openerp-server -u yourModuleName.
For example if you want to update the module sale it would look like this:
2. Open your browser, surf to your Odoo instance and open the module you've upgraded.
You ''ll see it is upgraded.
Upgrade