You may already know, but when you play you compile experience points (XP) and credits (coins). You use the experience points to research upgrades to your tanks. Then you use credits to buy the upgrades.
By way of example, with your Tier 1 tanks. You will note that each time you play a battle, and return to the garage, the number of XP and coins is increased from the last time you looked at that tank in the garage. Each tank starts out with stock components (chassis, engine, turret, radio, gun). When your tank is in the garage, hit the research button. That will show you what you have loaded on your tank. It will also show you how many XP are required to unlock the next higher chassis, engine, turret, radio or gun. Let's say you want to upgrade your chassis. And let's say you have compiled 300 XP for that tanks past battles. And let's say that the next chassis upgrade costs 250 XP. You can use 250 of your 300 XP to unlock that upgraded chassis. Then you have to buy that upgraded chassis with the credits (coins) you have amassed. Let's say the new chassis cost 1,000 credits, but you only have 925 credits built up. Then you cannot affort the chassis yet. Go ought a play another batte or two and you will get more credits, allowing to you purchse the upgraded chassis. Presto!
As a practical matter, you want the better chassis for a few reasons. First, it can take more damage before you go immobilized. Second, often times you need the upgraded chassis to carry the load of the bigger engines, upgraded turrets or consumables (vents, cammo net, gun rammer, etc.)
Hope this helps.