July 12th 2021 edit: There is now an ‘Increase Neuron Stake’ button on each neuron in the NNS app which makes this whole process much easier!
We have just updated the NNS app (https://nns.ic0.app/).
In this new version, if you want to add ICP to an existing neuron, all you have to do is send ICP to the address that the original ‘Stake Neuron’ transaction went to and the neuron’s balance will be refreshed automatically.
Here’s an example of a neuron I just staked -
In this example you can see that the ‘Stake Neuron’ transaction was sent to this account -
So to top up that neuron you can simply send more tokens to that same account.
Please note this only currently works for neurons created since May 19th since it requires the memo values which we did not store in the NNS app before that date. A fix for this will follow shortly (hopefully next week).
You can tell if the auto top up will work for your neuron by looking in your transactions list, and if the transaction is labelled as ‘Stake Neuron’, then the auto top-up will work, if not, it will still need to be done manually until the fix mentioned above goes live. For those that need to be done manually, please drop me a message and include your principal plus the account that the stake neuron transaction was sent to and I’ll refresh the neuron’s balance.