How to fix the pages marked with noindex?

Hello, I checked my google search console and found there are 1385 pages have been marked with noindex. I watched some teaching videos on Youtube saying I need to open source code and find "<meta name="robots" content="noindex"" and then revise it into "<meta name="robots" content="index"" , but I tried to open the page's open source code but didn't find it. How could I do it? Please help, thank you.