I've been doing only iOS development and never made an OSX app. In the iOS project I'm working on now I see that suddenly Xcode thinks it's an OSX project. The all my framework files are shown in red (missing), and lots of the options in the project panel show choices that are only appropriate for OSX such as "deployment target", which offers only 10.4, 10.5, 10.6 etc. and in Build Settings the Valid Architecture shows i386 x86_64 with no drop-down options.
What happened? How can I change this back to an iOS project?
could be 2 late to answer this question, however, this answer might help someone in the future, as the above answer wasn't helpful for me, so here is what I have done.
1- on the left top corner of Xcode click on the scheme.
2- click edit scheme
3- in the new window change "Executable" from non to the name of your project scheme "target"
4- on the top where it says "My Mac 64-bit" if it didn't changed automatically to iPhone/iPad change it manually.
5- if there wasn't any option, in bottom of the window click "Manage Schemes" and make sure the check box next to your project scheme is checked.
hope this could help and save anyone time.
By your comment you should not have an issue navigating this.
Go to your Project (in upper left) -> Project (project name in submenu) -> Build Settings -> under Architectures menu select Base SDK and choose iOS 6.1.
This should switch your project back. Now under Info you should see your Deployment Target options have switched from saying OS X Development Target to iOS Development Target
As for why it originally changed, I have no idea. But hopefully this will fix your issue.
I know this is an old post, but this just happened to me in a project with multiple targets.
I'm using github and it had marked one of my target scheme files with a merge block at the top of the file after using a newer compiler. I could find a way to edit the scheme through XCode, but upon re-opening the project, it would be bad again.
I had to edit the file manually and complete the merge edit.
This recently happened to me with Xcode 8 where an old iOS project was turned into a dual destination macOS / iOS one.
To fix the issue I changed the Base SDK under Project -> Build Settings to Latest iOS (iOS10) and the change rippled through removing the macOS destination.
来源：https://stackoverflow.com/questions/17984469/how-did-my-xcode-project-get-changed-from-ios-project-to-an-osx-project