BigCommerce does not provide an easy way to make bulk title tag changes in their admin. However you can update your theme files to dynamically set the title tag based on the template being used. Below is what I have used with custom stores I have built.
<title>{{#if template '==' 'pages/product'}}{{product.brand.name}} {{ head.title }}{{else if template '==' 'pages/category'}}Buy {{category.name}} Online - {{settings.store_name}}{{else if template '==' 'pages/brand'}}Buy {{brand.name}} Online - {{settings.store_name}}{{else if template '==' 'pages/home'}}{{ head.title }}{{else}}{{ head.title }} - {{settings.store_name}}{{/if}}</title>
