A theme is made up of templates. The theme is a set of templates.
Templates are a part of the theme. for e.g. In a house, each room has a specific decoration, layout, colors, etc. But overall there is a theme for a house. Each room is part of the house and the decor of each room is according to a particular theme.
You can change the theme of the entire house in which case the entire theme is updated including the templates Or you can only update the decor of a specific room. In that case, the change affects the template only and not the entire theme.
You can change the WordPress theme from the WordPress dashboard. Just go to Appearance>>Themes.
For changing a template you will need some knowledge of code or PHP to be precise. In order to change a template for any page for example you need to first create a child theme and then create a page template.
You can learn more about Page templates from the official WordPress website.