That may lead to some astonishment - it changes all occurrences of "-15" TO "-2035". Maybe you should just change the data you need to the data you want. Note the 3 after the single quote restricts the update to only the third line.
sed -i '3s/-15/-20/' filename
There are also simple script solutions to iterate over a director structure to do this automatically in (applicable) all files, but requires knowledge of how you have said directories organized.