I have $900k+ in my companies stock, and I'm quitting in 5 months...

I have $900k+ in my companies stock. I'm planning on quitting in 5 months. My companies stock is at an all time high right now. I have mostly NSOs.

What are things I should consider for when I sell my stock? Is there benefit to spreading it out over 2 tax years?

I don't know anything about buying or selling stock, so I really don't know what I should be doing. I also, more importantly, don't know what I don't know. Any advice you give me would be appreciated.

EDIT:

1. The stock is UNH.
2. I have primarily vested NSOs, maybe some ISOs, and some RSUs. My company gave them to me, and I had to stay at the company for 4 years for them to vest. Now I can buy them at the strike price (the amount they were awarded to me at, which happens to be significantly less than they are worth now) and sell them at their current price. In order for me to keep them, I'd have to sell some of them so I could afford to buy the rest. A small percent of what I have is RSUs. I will keep those.