You can rename any project by right-clicking the project in Solution Explorer and select Rename. But this doesn’t change the physical folder name. To change the physical folder name, first right-select “Open Folder in Window Explorer”, then close the solution (File> Close Solution), then rename the folder that contains the project (the folder in which the *.csproj file resides).
The assembly name and the namespace of the projects under this solution still remain unchanged (suppose you’ve changed namespaces and/or assembly names for some of them), to fix this, while the solution is still closed, open their individual .csproj file with a text editor and modify the <RootNamespace> and <AssemblyName> nodes to reflect the changes.
Last but not least, When you reopen the solution, VS won’t be able to load the project. To fix this, remove the stale project entry in Solution Explorer, then right-select the Properties of the solution > Add > Existing Project and navigate to select the *.csproj file.