How to find .NET version of all projects in solution

Short PowerShell script:

$solutionFolder = [solution folder]
Get-ChildItem $solutionFolder -Include *.csproj -Recurse -Force | ForEach-Object {
[xml]$projectXml = (Get-Content ($_))
$namespace=New-Object System.Xml.XmlNamespaceManager($projectXml.NameTable)
$namespace.AddNamespace("nsp", $projectXml.DocumentElement.NamespaceURI)

$dotnetVersionNode = $projectXml.SelectSingleNode("//nsp:TargetFrameworkVersion", $namespace)
if ($dotnetVersionNode -eq $null) {
$dotnetVersionNode = $projectXml.SelectSingleNode("//nsp:TargetFramework", $namespace)
}
Write-Host $_ : $dotnetVersionNode.InnerXml
}

Note: This script will also identify targeted framework versions for .Net Core projects

You didn't google hard enough.

This is what you need: Target Framework Migrator - Visual Studio Marketplace[^]

In the csProj or vbProj file for each project will be a line that looks like

<TargetFrameworkVersion>v3.5</TargetFrameworkVersion>

You could use a text search in files tools (such as the one that comes with the free editor NotePad++[^] to search for the phrase TargetFrameworkVersion to get a list of the versions in the project

One way is to write a quick app.
Each project within a solution is inside it's own folder below the main solution folder, and each project folder contains a ".CSPROJ" or ".VBPROJ" file.
That project file is an XML descriptor of the project, and contains a node PropertyGroup/TargetFrameworkVersion.
If you scan each folder, read each project file, you can pick up (or change) the framework version for all projects in the solution. Note that changing the framework version may cause compiler errors you'll need to fix, but you probably know that already!