To Upgrade your solution from a 2010 project to 2013, you need to follow these steps
First edit your .csproj file (for c#).
Modify the target framework to this:
Add this a node for the office version, I put mine directly below the TargetFrameworkVersion tag
Reload the project and update your referenced assemblies. If you haven’t specified a specific version they should already be referencing the v15 (SharePoint 2013) assemblies.
Do a find replace for
18.104.22.168. This updates any references on your pages, layouts, and master pages to the v15 assemblies.
Change any calls to
Check each file to do a check for any hive references. You’ll need to add a /15/ to these. EG: _layouts/ to _layouts/15/
Open the package “folder” in visual studio then update the properties for that package to use version 15.
Finally do a compile clean up any missed items. Deploy your solution and make sure to test thoroughly.