Steps for de-/soldering BGA chip
0. Take note of pin 1 marking!
1. Mask off surrounding chips (kapton or alumium tape).
2. Add flux, start with 200 ℃ @ 60% air flow to warm up the board a bit.
3. Turn up to 400 ℃ to finish desoldering unleaded components, add new flux if needed.
4. Reball new chip with leaded solder.
5. Clean pads on PCB: Apply flux and use a soldering iron with a big tip + ball of leaded solder @ 360 ℃ to clean pads.
6. Use solder wick to remove remaining solder.
7. Clean with IPA.
8. Redo 6+7 as needed.
9. Apply a thin layer of flux to the PCB, use a cotton swab to spread it out.
10. Align IC perfectly (remember pin 1 marking from step 0!).
11. Use hot air at 360 ℃ / 40 % air flow (so the chip does not fly away).
12. Gently knock a corner to see the chip jumping back into position to know when the solder is melted.
13. Let it cool down slowly.

https://www.youtube.com/watch?v=5pW4_nqcIWA