If it's escrow, you ping the freelancer and ask them if it's cool to add another milestone. If he/she says it's cool, then you add a milestone and fund it.
If it's hourly, you ping the freelancer and discuss what you need and they just bill it to the contract.
Note that it is TECHNICALLY possible for a client to add a milestone without talking to a freelancer at all.
But as a freelancer... let's just say I REALLY appreciate if clients talk to me first. If a client has some kind of understanding with the freelancer about adding milestones without consultation, that's fine. But the default behavior for clients should be to never do that.