There are three types of inheritance in Odoo. If you want to modify an existing model you should use the "_inherit" attribute and set it to the model you want to modify.

e.g. _inherit = "model.name"

Make sure that you do not set a value to the "_name" attribute since this would create a new model with that name and have the same properties as the model mentioned in the "_inherit" attribute. If you do not use "_name" it will modify the existing model by the name mentioned in the "_inherit" attribute.

Thereafter whatever the columns you define will be added to the existing model and whatever the functions you define will alter the behaviour of existing model.

Hope this helps. :)

See in documentation how Build an Odoo module: https://www.odoo.com/documentation/8.0/howtos/backend.html