If you want to top-up or add ICP utility tokens to an existing neuron, you have to send tokens to the underlying sub-account address associated with the neuron. This can be found on the NNS dapp by selecting the neuron in the Neuron tab.
On Quill, you have to use the "list-neurons" command.
Once the ICP tokens are sent, the neuron’s balance will be refreshed automatically.
Please refer to this forum guide for more information.