In 5E, you don't increase the maxima. The maxima always stay at 20. (In the standard rules, that is... you could always apply house rules if you like.) In 5E, you build racial packages by increasing the base value. So, for example, if you want Half-Giant as a race, you need to decide what you want their average STR to be and (if necessary) add or subtract that much STR from the package deal. Let's say you want the average Half-Giant to have a STR of 18. You put "+8 STR" in the package deal for 8 points, and all Half-Giants now start with 18 STR instead of 10 STR, and raise it or sell it back from that point.
However, they still start paying double cost for STR once it reaches 20. Yes, this would mean that a Half-Giant could only buy two points of STR before bumping into the double-cost rule.
The reason this changed in 5E is that the old 4th Ed. way was unfair. It charged you points for being able to exceed a certain value, even if you were actually nowhere near that value. If you read the 4th Ed. rules on this, you'll see that a 30 STR character with increased char maxima that allow a 30 STR with no cost doubling, and character with no such change that had to pay double points for their STR from 20 to 30, actually end up spending exactly the same amount of points for STR! And if the Half-Giant (or whatever) didn't buy his STR up to 30, then he ended up spending points for something he didn't have! And this whole system didn't make the average Half-Giant any stronger than the average human.
The 5th Ed. way is much better.