Flat file master data upload

Hello,

I am trying to load master data through flat file. However there is a property in this dimension that is not being loaded from flat file. That property is getting overwritten. Here is an example:

Account ID       Property - Region

1900                         ABC

So when I load the new flat file that has account 1900(property Region is not in the flat file), it overwrites the property. Any advice how I can solve this issue?

Thanks.