SDFAT single line save when powered off

I am using the excellent SDFAT library to log data to a file and separately to read a CSV file line by line. I have used examples posted on this forum and I can nearly understand what I am doing ! but I have a long way to go. What I need now is to be able to use another dedicated SD file to retain two floats during power off. I think I need to open the file read a value modify the value in the sketch delete the old Sd value and then save the new value and the same for the other float. Can anybody point me in the right direction to do this. Any examples or code snippet or tips greatly appreciated. Thanks and regards Geri

open fil for read - read values - close file
open fil for write - seek pos 0 - write new values - close file

or store in EEPROM

Many thanks for the reply. I think I need to add to the basics the steps of converting floats to char both in and out. I am by definition getting closer to a solution and I think one of the problems I have been struggling with is my lack of understanding of serial in and out rather than the specific sd SDFAT library functions. I have just read the brilliant serial basics thread from robin2 on this forum and this has given me food for thought. I'll keep plodding on in the knowledge that mistakes are as useful as successes as long as you learn from them. Regards