How to change blog layout using Helix Ultimate 2?

Hi.

Helix Ultimate has its own override system for those files that overriden in template from the box . Look at the documentation.

But for your task, there is no need to create override. CSS is enough.

For example

.article-list .article {
    display: grid;
    grid-template-columns: .4fr 1fr;
    gap: 1em;
}
.article-list .article .article-intro-image {
    margin: 0;
}

Result