How to change the excerpts length from the categories page ?

Hi,
Solution1: To resolve this issue, you can log in to Dashboard > Posts > All post,
Hover over your post title and click "edit" below the title. Click on the Text view tab instead of the standard WordPress Visual Editor tab.

• Find the insertion point for "Read More." It is a button on the toolbar that says "More"
• Place your cursor at the point in your post where you want to end your excerpt and put the "Read More" link.
• Click on the "More" button. This will insert the "Read More" code. You should place the link at a point where the reader knows what the post is about. It should also entice them to read more. Try to place it at 2 to 3 paragraphs or less into the article.
  See the screenshot: http://prntscr.com/8c1gk9

Solution2:
You can add the following code to the functions.php file:

function get_excerpt($count){
  $permalink = get_permalink($post->ID);
  $excerpt = get_the_content();
  $excerpt = strip_tags($excerpt);
  $excerpt = substr($excerpt, 0, $count);
  $excerpt = $excerpt.'... <a href="'.$permalink.'">Read More</a>';
  return $excerpt;
}

Then open the content-archive.php file. Find the line 98:
Replace the following code:
<?php the_excerpt(); ?>

With new code:
<?php echo get_excerpt(45); ?>
Hope this helps !