You could be tempted to buy a business class ticket and then upgrade to first class later. A business class ticket can be five times the price of a coach ticket. While flying in business class is preferable, it should not five times expensive. Plus, you'll almost certainly have to pay a lot of money to fly first class.
Purchase the Points
There is a slew of websites where you may purchase and sell points, but stay away from them. The method is frowned upon by major airlines, and you may lose your miles or be unable to use the miles you acquired as a result.
Upgrade at the time of check-in
If you truly want an upgraded seat and don't have enough miles to acquire one for free, buy one when you check-in, even if it's online. Because there are barely 24 hours to take off, airlines frequently offer cheap tickets if seats are available.