ODOO DDBB Upgrade v.13 to v.15 or v.16

We want to upgrade an existing ODOO v.13 Enterprise edition to a v.15 or even v16 as there are no custom made modules and all modules we use from Odoo are available in v16. The current v.13 is fully working except for a tax reports that seems to have a problem. No third party modules are installed except for a Theme. It’s a multi company site, 3 companies.
We need it ASAP.
Would be helpful to get a price or hours estimation.
Thanks!