To top up a customer's account, go to the "Billing and Collection" section in the customer's file and to the "Balance" tab after.
There you can top up the client's account and can also correct the amount, see all movements done etc.
Once inside the "Balance" section, press on the "Recharging Balance" button to proceed.
In the top up window you can choose between the options you have (you can create rules, for example if someone pays 100, they get 120 instead as an offer) or select personalized and write the amount paid and the amount to top up.
Once you're done press Recharge balance.
If you want to create different options to select (the rules mentioned before) Go to: Customers -> General Fees -> Recharge Types. There you can create the options you want to appear in the dropdown menu when you top up an account.