It's not an elephant in the room. They misrepresented their experience and qualifications, and now they're flirting with gross incompetence.
I'm assuming you don't have an HR department because why on earth would you be handling this and asking the internet if you did (LARPing aside), but if you want to go through the motions, notify them of their under-performance, ask them what resources they need to perform to the expectation of the role as applied for (which you also make clear and document), bring that to whomever for approval, and then set progress improvement checkpoints every week to see where they're at. This could be training, additional staff support, consulting, etc. Give the employee a week to decide and put together their proposal.
If they don't get the picture and start applying elsewhere while they're still employed, then evaluate them per the agreed timeline or, if the resources aren't approved and the project can't be delegated, go ahead and terminate their employment.
Otherwise, and this is more mature, better, kinder way to handle it, just let them know they weren't a good fit, don't apologize but do offer them a generous severance, and let them go. Or if you want, you can hire me for a day, and I'll do it.