Start here. Learn about unions.
Then create a union what contains a float and a byte array:
union
{
float f;
byte b[4];
} u;
To write a float:
u.f = valueToSend;
Serial.write(u.b, 4);
This will require that you reassemble the bytes in the same order on the other end. Read 4 bytes, storing them in u.b[0], u.b[1], u.b[2], and u.b[4]. Then use u.f for whatever.