Can't change difficulty level

Hi!

I've faced the issue - once a STAGE is COMPLETE, I no longer can change the difficulty of the questions. While reviewing, I can choose the option to practice SIMILAR (i.e. BASIC) questions, which all are repeated, and can not set medium or difficult level (since initially the level was basic and I didn't change it because I wanted to raise the level gradually). So how can I return after completing a stage to the menu with the yellow bar where I can choose another difficulty of questions?

Thanks in advance,

Evgenia