The President does not have the power to change rules governing elections via executive action. The Constitution gives Congress limited power to determine how elections are conducted, outside of setting an election date and requiring the creation of congressional districts. Various amendments allow Congress to pass election-related laws for specific purposes.
The only recourse for a sitting executive seeking to challenge rules is in the courts. The Trump campaign and the Republican Party filed a lawsuit in Nevada on Aug. 4 seeking to block a new state law enabling mail voting.