The best advice is to consult the documentation that came with the theme (it's a commercial theme, offered by themeforest. Commercial themes are not supported here, but rather wherever you made your purchase) and see if the theme has any options in the dashboard that simply let you upload a new logo from there. A lot of themes come with that option. If not, the simplest solution might be;
Your current logo is located here: /wp-content/themes/complete-wp/_layout/images/logo.png
If you rename the old logo.png file to something like old_logo.png, and then name your new logo "logo.png" and place it in the same location as the old one, that should show your new logo. The new one should be the exact same dimensions as the old one, and should have the .png extension. Otherwise you will need to edit your style sheet or your header.
You should however, consult theme support at themeforest for issues with the Complete WP theme.