As you're on SSIS 2012, you should consider creating an 'environment' which contains your connection strings and use the 'Configure' option to map those connection strings to connection managers in the package - or even at the project level.
Here's a link to introduce you to the topic.
If you haven't even tried to resolve your issue, please don't expect the hard-working volunteers here to waste their time providing links to answers which you could easily have found yourself.