My google search console gives me those warnings. I'm using the "Supply" theme. But I find many other themes have the same problems. This is no solution from Shopify now(according to their answer in this one).
I just want to know how to add structured data to our theme (microdata & JSON-LD) and remove the old one. I'm not familiar with the code, I hope someone can teach us step by step.
Thanks a lot
Learning to edit your theme AND trying to get a theme specific solution AND running your business at the same time will only create headaches.
Break it down read shopify help docs on editing and designing themes, and liquid. The post you linked has tutorials linked that can be adapted once your more comfortable editing themes. Then revisit this or hire someone to train you or implement it or use an app.
Where you need to put specific code for specific purposes vary by theme,
Here most schema or structured data either goes into the <head> area of your theme.liquid or into product.liquid (or related sections) and often needs to use liquid variables to fit