Editing a variable in a file

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:bust_in_silhouette: Asked By tayete

I cannot seem to be able to find any clues about this topic around.
I need to read a variable from a file in which it is stored, change it value and store it back again. I have achieved opening the file, changing the var and closing the file, but the rest of the file’s content is lost.
Anyone knows an example of how to update just a part of a file keeping the rest intact?
Thanks in advance!