In World of Warships Blitz, you can upgrade the characteristics of your ship with Blueprints. The upgrade screen can be found in the armory.
There are 5 Levels of Blueprints, indicated by the star symbols. Level 1 blueprints are for Tier 1 and 2 ships, Level 2 blueprints for Tier 3 and 4 and so on. In the Warehouse, you can check how many blueprints you have in stock.
You can get Blueprints from crates or as a reward after each battle.
As soon as you have fully upgraded your ship, you can choose an Elite Ship Bonus. This Bonus will give you an extra boost. Choosing the bonus the first time is free, but further changes will need to be paid with gold, so choose wisely!