If you would like to change the start time of one activity event you will go to the diary, click on the class and click the start time now you can change this 😊
If you would like to change the time of a class which reoccurs you go to the ongoing schedule, then click on the class. now change the time at the top of the screen.
Then click 'update existing activities' and 'update'
Link attached will take you to the article which will show you have to delete a reoccurring calls