How to set an EXIF tag value?

After setting up bits, reading the file, etc... I do following:

meta.SetProperty_Float(kXMP_NS_EXIF, "ApertureValue", 0.8, 0);

if (file.CanPutXMP(meta))

{

     file.PutXMP(meta);

}

, and then close the file and connections.

After this I suppose to overwrite the original value in the ApertureValue, but when trying to get it next time as follows:

std::string someString;

meta.GetProperty(kXMP_NS_EXIF, "ApertureValue", &someString, 0);

I have the old result (in my case 1/1) in the someString.

The same strategy works fine for Date&Time tags.

Am I doing anything wrong, or it is supposed to be so? If so, what is the right way to overwrite it?

Thanks,

Mikayel