To change the font size in HTML/CSS, you can use the following methods:

• Inline CSS:

<p style="font-size: 20px;">This text is 20 pixels.</p>

• Internal CSS:

<style>
  p {
    font-size: 18px;
  }
</style>
<p>This text is 18 pixels.</p>

• External CSS:
  Create a CSS file (e.g., styles.css):

p {
  font-size: 16px;
}

Link it in your HTML:

<link rel="stylesheet" href="styles.css">
<p>This text is 16 pixels.</p>

You can use units like px (pixels), em, rem, %, etc., depending on your needs.