The correct way of upgrading any nopCommerce installation is to first download the upgrade scripts
for all the subsequent versions. So for 3.7 version, you should download all the upgrade scripts 3.8,3.9,....upto 4.3, and run them one by one via SQL Server Management Studio.
What this does is that it makes sure that you are actually adapting all the changes that those versions made to the database. Once you do this, your core database will be upgraded.
Next you should upgrade plugins by consulting with your plugin vendors, as each of them may have different ways of doing it.