This article from PSL [Party for Socialism and Liberation] provides an accurate explanation of who the "New" President is, and why the for those who don't follow this news regularly.
A brief explanation is that "The National Assembly was declared to be in contempt by the Venezuelan Supreme Court after it defied the National Electoral Council and seated legislators whose elections were nullified on the basis of vote buying. All decisions made by the National Assembly, including the selection of Guaidó as its leader, are unconstitutional and null and void."
If i remember correctly, that Court Decision was made in 2014 or '15