To configure properties at run time use a configuration file.
To specify the location of the configuration file use an environment variable


To create the environment variable

control panel, system, advanced, Environment Variables
under system variables click new and create the variable 
e.g.
variable name = MySSISConfig
variable value = c:\ssis\myssisconfig.dtsconfig

Now reboot the machine to make the environment variable and value available.



Add the configuration to the package

Open the package in the BI IS designer
SSIS, Package configurations, Add, next
select XML configuration file
select Configuration location is stored in an environment variable
Type in the environment variable name (e.g. MySSISConfig)
next
Type in the configuration name e.g. ConfigFile
Finish


The package will now use the configuration file referenced in the environment variable.