Open your save file, change your coins, unlocks and progress, then download the edited save and load it back into the game.
In the browser game, choose Save data, then Import backup. The downloaded game reads a plain save.json instead (see Install for where it goes). Either way, importing replaces the save on that device, so keep a copy of the original. To keep your real progress, settings, loadouts, custom tanks and story progress, load your own backup or save.json below and edit that. Everything runs in this page; nothing is uploaded.
Starting from a blank save. You can also drop a file here.