Balls can be upgraded in the Arsenal. You need chips and ball cards to upgrade a ball.
The maximum level for all balls is 18. Keep in mind that after reaching the maximum level for a Ball you will keep getting its cards. These cards will be converted into the chips added to your account.
If you have enough cards to upgrade a ball, there will be a green arrow pointing up next to the progress bar on the ball.
By tapping on the ball, you'll see how many chips are required for the next upgrade.
The light gray part is the current value of Weight / Speed / Guide / Hook / Hook Control. The green part is the value after the upgrade. The dark gray part shows the value at the ball's maximum level.