You can take measures to reduce the downtime caused by upgrade of the MS SQL Server database. Normally, the SQL Server database upgrade procedure is very fast.
You can find more information on upgrading the SQL Server database at:
You read the upgrade documentation.
This helps to avoid unexpected problems during the upgrade.
You plan the upgrade carefully.
Rather than simply starting the upgrade, plan the procedures involved in advance. Make sure that all the resources are available.
You rehearse the upgrade using a test system.
The best preparation of all is to rehearse the upgrade fully using a test system that is as similar to the production system as possible.
You can now upgrade the database with the minimum possible downtime.