change WHOLE curve interpolation in graph editor

Hi I am trying to change whole curve interpolation in graph editor.
When I select the whole curve it changes only 1 (one) keyframe interpolation
from (for example) constatn -> linear.

I had tried selecting all keyframes (AA) on it or by marking it (b). All the keyframes were selected yet
only one of them had changed it’s interpolation.

tnx in advance for the answer.

select the curve
key>interpolation mode (shortcut “t”)