Please how can i fuse champions 

You need to make sure you have all the required champions, leveled and ascended to the requested level, and unlocked. Make sure you remove any gear that you want to keep as well. Then you'll be able to select them in the fusion menu. Once you've selected all the required champions in the fusion menu, you'll be able to hit the fusion button, though it will also cost some silver (a few hundred k for fusing an epic, 1 million silver for fusing a legendary like Rhazin).