>Ok, I'll take that risk, anyway i will not implement it right now but i need to do some research on it. Do you know how?

There is pseudo-code. You'll have to "fill blanks" to get it work

lcMyTable = "MyTable.dbf"
lnFileHandle = FOPEN(lcMyTable, 12) && Open Read/write
IF lnFileHandle < 1                 && Check if there was error
    * Cannot open file - do something
ENDIF

lnOffset = 0                       && An offset of byte you want to overwrite
lcNewVal = "Z"                     && The new data 
= FSEEK(lnFileHandle, lnOffset, 0) && Position file pointer
= FWRITE(lnFileHandle, lcNewVal)   && Write new data
= FCLOSE(lnFileHandle)             && Close File