How do I solder that? The solder won't stick
Figure 1. Detail from OP's photo.

There is no sign of a soldering attempt on this joint.


Apply the solder-wetted soldering iron tip to the PCB pad and the pin simultaneously for a couple of seconds to heat it up.
Feed in a suitable guage solder with flux core until the joint and the pad are wetted.
Withdraw the soldering iron and allow the joint to cool.