How to handle different environments?

I am using SSIS 2012 to develop an SSIS package which will be deployed to DEV, UAT and Production environments.  Could someone walk me through how to handle the different connection strings needed for each environment?  My connection string is localhost in my package - but how can I easily change this for each environment?

Should I add a config file somehow and deploy a different config to each environment?  Note that the SSIS package will be on the file system and will most likely be copied over during a build.

I have been reading a lot about environment variables but can't seem to find them in SSIS 2012.

Looking for a simple guide.

Thanks