how to buy sims 4 standard edition after purchasing the legacy edition

Technically you have that backwards; Legacy is free with your purchase of Sims 4. Legacy Edition is a free "downgrade" for existing users to install the game on their sub-par PCs. If you already are able to play Legacy Edition then you /already/ own Sims 4; all you need to do is redownload it and uncheck the "Legacy Edition" Expansion content.