Question about stock capital gain tax

Hi,
I have two questions about stock capital gain tax.
1.Can I pay capital gain tax as soon as I sell the stocks (for example I sell stock in October 2021) and how (can I use amended tax return)? Because it is very hard to estimate how much gain or loss I will get and I don't want to pay penalty and interest in next year.
2.If I exercise a stock option and hold that stock (I bought stock option from market, not employer incentive stock option), do I need to pay capital gain tax?

 

Thanks

 

James