To edit frictions coefficient of the current reach click in menu Hydraulics > Edit friction.
The geometry edition window contains a table with the list of friction section defined by the begin KP, end KP and two Stricklers coefficient.
Define Stricklers coefficient
To edit the Stricklers coefficients list click on edit button , a new window will appear. In this window, you can define new Stricklers coefficients, the name and comment are optional.