You can buy premium content from the store such as coolant and new Mechadroids. More information can be found at the store page.
Upgrading your weapons increases your Mechadroid's damage (not its chance to hit). Weaponless, your Mechadroid's base damage is one. When you purchase a weapon, the weapon's damage is added to the base damage of the Mechadroid. You can only have one weapon equipped at a time.
Upgrading your shields increases your Mechadroid's damage absorption (not its chance to defend). Shieldless, your Mechadroid has a base absorption of zero. Absorption reduces the damage taken from each successful attack from your opponent. You can only have one shield equipped at a time.
Each Mechadroid has ten points of armour to spread over the six Mechadroid parts (head, left arm, body, right arm, left leg and right leg). In refit you're able to change how this armour is configured and this allows you to keep your opponent guessing your weakpoints!
When you level up you are able to upgrade your attributes. More information can be found at the upgrade page.
If you're tired of how your Mechadroid looks you can purchase a 'rebuild' premium item or a premium Mechadroid. This allows you to rebuild your Mechadroid with new parts and repaint it similar to the creation area of Mechadroids.