Hi Jorge,

At the moment you have two 'easy' options. You can upgrade through upgrade.odoo.com or through OpenUpgrade.
If you want upgrade your Odoo V9 to V10/V11/V12 and have an enterprise version or want to buy an enterprise version Odoo will upgrade your database for free. You can find more information about this at https://upgrade.odoo.com/database/upload 
Alternatively, if you don't have an enterprise license and you do not want one you can use the open source alternative OpenUpgrade. OpenUpgrade is made and managed by the Odoo community and allows you to upgrade your database(s) too but you'll need to do upgrade yourself. You can find more information about Openupgrade at https://github.com/OCA/OpenUpgrade and you can find the official documentation at https://doc.therp.nl/openupgrade/ 

The only part that remains is your custom development: you will need to migrate these from V9 to V10/V11/V12. You can migrate the code yourself, reach out to an Odoo partner or pay Odoo to migrate the code for you.

Regards,
Yenthe