To change the prices for your client, you need to go to the user management menu in the Admin menu -> Users (1), then go to the service tarrifs (2) icon
Next you need to choose for what type of messages you want to change the price, in this case for SMS and click on the price icon.
If you need to add a new price, then you need to click on the "Add" button and fill in all the fields, making the price higher than your current (1).
Change existing price (2)
View price history (3)
Select multiple values and delete unnecessary (4)