Steps:
1. Make a backup of everything on your site, including the database. This is extremely important so that you can roll back to a running site no matter what happens during migration.
2. Execute upgrade.sql script over your database
3. Remove all files from the previous version except App_Data\dataSettings.json (Settings.txt) and App_Data\installedPlugins.json (InstalledPlugins.txt)
4. Upload new site files
5. Copy back App_Data\dataSettings.json and App_Data\installedPlugins.json files
6. Ensure that everything is OK

Notes:
1. If you stored your pictures on the file system, then also backup them (\wwwroot\Images\) and copy back after upgrade