delta loading throgh flat file in 7.0

I am loading data from flat file in to cube I am working on 7.0,first I am loading data in to PSA & from PSA to cube using DTP

Now I want to know whenever we load data for the first time we select the option full update in DTP

When I am loading the data for the second time even I can see full update option

Again in psa if I am loading data from another file then then it fine & from psa to datatargets when I am loading through DTp again my DTP is selecting 2 data packages of PSA first & second one but I wan t only to take the 2nd datapackage i.e delata

How can this be done

Even I have slected the option delta update in DTp