Firstly you need to add NuGet feed for Sitecore 9.3
Install-Package Sitecore -Version 9.3.0 -Source https://sitecore.myget.org/F/sc-packages/api/v3/index.json
Then you need to go Manage Nuget Package
and search DLL name then click on update
If you want to update multiple projects, then click on "Manage Nuget Packages for solution" and type dll name and check which project you want to update.