There is no right way to re-position popover after it was created but you can call popover('show') each time you need to change position and do all your calculations inside placement function.

jQuery(document).ready(function() {
	jQuery('#popover-element').popover({
		content: jQuery('#popover-content'),
		placement() {
			if (jQuery(window).width() <= 992) {
				return 'top';
			}

			return 'left'
		},
		html: true,
		trigger: 'manual',
	}).popover('show');

	jQuery(window).resize(function() {
		jQuery('#popover-element').popover('show');
	});
})