Browse to the folder you installed osu! to (C:\Program Files\osu! by default) and open the "osu!.[username].cfg" file using any text editor, such as Notepad. The [username] part is the username for your account on your computer, not your osu! account.
Once open, find the line that starts with
Once open, find the line that starts with
Renderer = ###and replace ### with the other value:
Renderer = d3dSave the file and restart osu!.
Renderer = opengl