How to modifiy an activity from Cycling to Walking?

Thanks for your answer. When I click on an activity I cannot see details. That's the problem. I found a workaround: I must go to reports and the I can select the activity and modifiy it

When I click on an activity I cannot see details

Not sure what you mean. Can you post a screenshot? As tmk2 mentioned, open the activity, then click the dropdown to the right of where it says "Cycling" then select "Walking"

Here's mine where I have the option to change the activity from RUNNING to Walking.

When I go to All activities, I cannot see the details of one:

Thanks for help. To select one activity, I go to "report menu" and I select the rignt one

Ummm, did you do a right-click? Just click with your left mouse button the title of the activity on the "All activities" list, and you'll get the details.