Upgrade Visual Studio 2013 solutions to Visual Studio 2015

The simplest solution IMO (also worked for 2012 and 2013) is:

1. Open the solution file using Visual Studio 2015
2. Select the solution file in Solution Explorer
3. Select File / Save MySolution.sln As...
4. Overwrite the existing solution file.

Change the version in the .sln file

# Visual Studio 2013
VisualStudioVersion = 12.0.31101.0

To match whatever version you have

As of this morning my VS Enterprise is 14.0.23107.0

Example:

# Visual Studio 2015
VisualStudioVersion = 14.0.23107.0

Visual Studio 2015 Update 3 is 14.0.25420.1

Note: This works for VS 2015 and 2017

An alternative to hand-editing the .sln file or re-saving on top of the original .sln file:

1. Open the solution in Visual Studio
2. Right click solution > Add > New Solution Folder (name does not matter)
3. Save solution
4. Delete the newly added solution folder
5. Save solution

The solution will now be upgraded.

I ran across this looking for the same thing. The accepted answer works, but I noticed some comments about not being batchable. I found an option for batching and I thought I'd share.

You can use the /upgrade option in devenv.com. This means it's batchable. For example, to recurse the current directory upgrading all .sln files (after backing them up), you could do this:

dir -Recurse -path ".\" *.sln | ForEach-object {
    Copy-Item $_.FullName "$($_.DirectoryName)\$($_.Name.Remove($_.Name.Length - $_.Extension.Length)).vs2013$($_.Extension)";
    & "C:\Program Files (x86)\Microsoft Visual Studio 14.0\Common7\IDE\devenv.com" /upgrade $_.FullName
}

I solved the problem by this: Right click on solution, "Retarget solution".(vs2013)