To fuse units to your main unit, tap Units->Fuse Unit and follow these steps:
1. Select the main unit to receive the fusing. This is the unit that you want to level up or the unit you want higher Brave Burst for.
2. Tap an empty platform/circle beside your main unit to select what units you want to fuse into it. These units will disappear after the fusing. You can select up to five units. Make sure none of your important units are selected for fusion.
3. Tap Fuse to finish the fusion process.
In order to increase the level of your units, you must fuse other units into it. Metal Parade units such as Metal Kings, Metal Gods and Jewel Gods give the most experience, especially if you fuse the same element (Fire to Fire, Dark to Dark). You can get Metal Units in the Metal Parade. Make sure to read the [Metal and Jewel Parades] article to learn how to access this important feature of the game.
You can also increase the Brave Burst level of your units through fusing. You should fuse units that have similar brave bursts (attack, support, recovery) and there is a random chance that this will increase or like the images above, use burst frogs to level them up instantly. For more details about leveling Brave Bursts, please read this Article.