I am using qibla app in android /smart phone during every prayer. In some waqt the direction in one point, in other waqt it deviates to some extent(several degrees). Allaah knows what is right, but I tried my best. Do I have to repeat previous prayer?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

You must search for the correct Qiblah direction by looking at the niches of the mosques in the country you live in, or by asking the Muslims around you who know the direction of the Qiblah for certain.

If this is cannot be achieved, and these applications are known to be accurate, then there would be no harm if you rely on them. A minor deviation from the exact direction of the Qiblah would not harm, because one is only obliged to face the general direction of the body of the Ka’bah; Allah Says (what means): {So turn your face [i.e. yourself] toward Al-Masjid Al-Haraam (the Sacred Mosque in Makkah containing the Ka’bah).} [Quran 2:144]

Allah knows best.