Where can I edit this at?
{$welcomeblock}
ACP -> Templates and Style -> Templates -> Your theme -> Header -> and then choose which welcome block you want to edit. Admin, guest, etc.
Better solution be edit language file. /inc/languages/english/global.lang.php - 34 line:
$l['welcome_back'] = "<strong>Welcome back, {1}</strong>. You last visited: {2}";