Use XP earned from defeating enemies to upgrade your statistics using the 'Shop' window. Approximately 505,000 XP is needed to fully upgrade one statistic.
One complete playthrough of the game will get you enough XP to upgrade one statistic. If not, replay any level to boost your totals.
It is recommend that you max out 'Life' first. Living longer will help you accumulate more XP overall as you lose 10% of your XP earned in that level with every death.
Regardless of which statistic you are maxing out, purchase 'Revive' from the 'Shop' window on the level select. It can be found in 'Secondary' > 'Powerup' and is essentially an extra life. When you die you are instantly revived with no XP loss. You can only have ONE Revive powerup at a time.
Thanks to JohnnyRock84, tastefulmrship