Hi Zakaria,

You'll first have to create a button in XML. If you need to inherit an existing view your code should look something like this:

<odoo>
  <data>
    <record id="some_id" model="ir.ui.view">
      <field name="name">your.record.name</field>
      <field name="model">your.model</field>
      <field name="inherit_id" ref="module.view_name"/>
      <field name="arch" type="xml">
        <xpath expr=//form/header" position="inside">
          <button name="change_text" string="Change text" type="object"/>
        </xpath>
      </field>
    </record>
 </data>
</odoo>

Otherwise you can simply add the button:

<button name="change_text" string="Change text" type="object"/>

Then you need to write the Python code to trigger the button click:

@api.multi
def change_value(self):
  self.your_field = 'new value'

If you would then click on the button the value of the field 'your_field' would change to 'new value'.

P.S: If you can already know the input or don't need a button click then use an @api.onchange on the field to change the value. The code would look like this:

@api.onchange('your_field')
def change_field_value(self):
  self.your_field = 'new value'

Regards,

Yenthe