<form>
    <header>
        <button name="action_do_something" type="object" string="Do Something"/>
    </header>
    <sheet>
        <field name="name"/>
    </sheet>
</form>

from odoo import fields, models

class TestAction(models.Model):
    _name = "test.action"

    name = fields.Char()

    def action_do_something(self):
        for record in self:
            record.name = "Something"
        return True