You can add or remove channels for a specific account by changing the 'Additional Channels' to the desired number of channels. The total number of channels for the account will be determined by 'Available Channels' + 'Additional Channels'. You can view the total channels in your Accounts page.
You are unable to change the 'Available Channels' as that is the base number of channels that comes with the assiged service plan.
To Add or Remove channels from an Account, from the Account list page, click on the 'Configuration' button. In the 'Additional Channels' field, enter the number of channels you would like to assign to the customer on top of the already assigned by the selected Service Plan. In the image below, this would mean assigning the customer a total of 15 channels.
Reducing channels on the portal is not available for Microsoft Teams Service Plan's Account.