2007 yaris rust repair

[HDS]

I have to weld some rust on this yaris 2007 under the door. Don't know what to call the exact spot english, but part of the frame of the car that is under the door, faces the road and is quite exposed.

Anyway on the bottom of this car there is a plastic list held on by plastic buttons. This has to come off so I can remove the rusted part and weld in a new one.

Is there a trick to removing these in a non destructive way? I'd rather not just rip them off.

Picture to help visualize where it is


IF someone could tell me the english name for this it'd be appreciated for future reference. Treshold I think is how it would translate, but it doesn't sound right.