If you are using the original Monimoto SIM card that came with the device and the home screen looks like this
You just have to go to the Monimoto app > Settings, press TOP UP SIM CARD and you will be linked to the shop where in just a few clicks you will be able to TOP UP your SIM card.
If you can't find the SIM CARD menu on your app and on the bottom you see [V1], it means that your Monimoto is running on an old cloud version and the firmware needs to be updated. Go here, follow the instructions and update firmware to the latest version. Once the firmware version is 0.10.5.61 or later, it will automatically apply to the new cloud [V2] and you will be able to see the TOP UP SIM CARD menu.