It depends how the template is being overridden, usually either a new template is specified in layout XML or a template is placed in the same path as the original but in the custom theme.
If you delete the template in your custom theme and re-deploy the core template should be used again.
We need a bit more info to help in more depth though:
- What version of Magento are you using?
- What is the template you want to revert
Update
To revert the checkout back to core this should be simple if the changes were done in a theme, if so delete app/design/frontend/{VENDOR}/{THEME}/Magento_Checkout
.
If the changes were done there your checkout should now be using the core templates/JS/styling.
Another option is the changes could have been made in a module, this is a bit tricker to find. You could look for files named onepage.phtml
or checkout-steps.js
and see if any modules are overriding them, or alternatively search for checkout
but that will return maaaany results.