You will need to figure out if it is stored in a string format ("123.45"), or in a binary format; and in the case of a binary format, how they are stored.
In the case of a string format, you read one char at a time and multiple it by 10 for each digit read and then process the dot separately. Or to use a library function.
in the case of a binary format, you typically use a union or a pointer to convert it back.