Things to consider for planned downtime

Hi,
If the business logic is contains at the DB layer, and navigation/validations/views is on the other tier what is the best way to manage and reduce the planned downtime.

In the planned downtime where in update is done to the database api’s and upgrade could have took 3 days.

Also,after the upgrade where the api’s get changes how to handle this scenarios.

Can somebody pleas provide me insight views on this?
Thanks