To massively change the levels of the clients they have in a sport we will access:

Tools --> Customers --> Game Level Change

We will open the following window, with the list of all clients:

In order to be able to change the levels we can do it or of a specific client, selecting it individually, or we can mark them all to make a change in general:

As the image shows, if we manually select several boxes from the first column on the left, we will be selecting only those clients. If we want a particular one, we look for the customer in the top box above and if we want to select them all with the "select all" button.

When we have the chosen option, we will go to the option "New level to apply" and we will introduce the new level that we want to apply to them:

We can also individually change the levels of customers they have in a particular sport.