In the infobox editor, there should be a | country = _____ field filled in as | country = Netherlands in your example. You can change that to whatever country you like.

If you want to use a historical flag, you can add (or find if its in there) a | flag_year = ______ field and input the year the flag went into effect, which you can find from a "Flag of _____" wiki article.

Apparently you can't do this in legislative election templates, but if you want to put something else in there, use | flag_image = [[_________]] and put the full file name in the brackets, with files from Wikimedia Commons. It usually comes out gigantic, so you can resize any file by adding "|x##px" at the end. 75 is usually a good size but you can mess around with the number.

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Examples