Please deactivate current UDesign theme in Appearance > Themes.
After deactivate current UDesign theme, simply delete the UDesign theme. Your content will be lost joking! It will never be lost.
Get u-design.zip file from the package you downloaded from Themeforest and simply upload it. To do it, navigate to Appearance > Themes and click Install Themes and upload zip file on it.
After uploading it, in Appearance > Themes, activate UDesign theme.
Don’t forget upload plugins, too.
Please go to Appearance > Theme Options in admin area and click Save Changes again.