Deadhead the dying brown flowers and the leaf as well (maybe some kind of bacterial/fungal infection started from a damaged leaf). Discard of the cut off parts.

Thanks! I did cut off the flowers that were crispy but I didn't cut off all the brown leaves because I was worried the plant wouldn't have enough for photosynthesis!

Usually new leaves will appear to make up for loss of old leaves. PS - these types of begonias are usually treated as annuals but may be kept year after year with the right kind of care. I quit buying them due to the extra care involved with these types (Rieger/Elatior/Hiemalis) and wish you good luck on sustaining them.