In general point of view, we upgrade to the next version by following 3 steps:

1. Install next version Odoo server

2. if you have any custom modules, migrate your custom codes to the target version

3. if you have any databases, migrate them to the target version as well.
   

Migration of custom modules is necessary, otherwise they may work with errors or do not work at all, and migration of database is necessary when you want to preserve data that already saved in old database if it was in production (i.e. if you have not production data in old databases, then you can skip this step and just create new databases in target version, but you can't use old databases without migration)

Here is some references for upgrading your codes of custom modules:

https://www.odoo.com/documentation/8.0/reference/orm.html#porting-from-the-old-api-to-the-new-api

http://fr.slideshare.net/openobject/odoo-from-v7-to-v8-the-new-api

And check the following link for upgrading your database:

https://www.odoo.com/documentation/8.0/reference/upgrade_api.html

You're welcome to find more references and post here.