Recomputing calculated fields has never been clearly documented in Odoo's developer documentation, and that's still the case in version 13.
Prior to version 13, to recompute a computed field you would flag the fields for re-computation and then call the recompute method on the model. Let's say we want to recalculate the display_name field on all res.partner records, prior to version 13 it would look like this:
partner_obj = self.env['res.partner']
self.env.add_todo(partner_obj._fields['display_name'], partner_obj.search([]))
model.recompute()
However in version 13 the add_todo method has been removed and the add_to_compute method has been introduced. There is no need to call the recompute method anymore.
The way to recompute calculated fields is now as follows:
partner_obj = self.env['res.partner']
self.env.add_to_compute(partner_obj._fields['display_name'], partner_obj.search([]))
Recalculate computed fields in Odoo 13
Odoo