Q:

How to only change the  tag that inside the  tag without having to modify HTML

I'm having trouble to make the p tag that inside of the ul tag to not have the same css modification and all the p tags not inside the ul tag.

ul,
li {
  list-style-type: circle;
  margin-left: 10px
}

p {
  margin-top: 0.2px;
  margin-bottom: 0.2px;
}
<html lang="en">

<head>
  <meta charset="UTF-8" />
  <link rel="stylesheet" href="style1.css" />
</head>

<body>
  <p>values is the world of hurt</p>
  <ul>
    <li>
      <p><abbr title="Internet Message Access Protocol">IMAP</abbr> is used to fetch mail from the server so you can read it in clients like Thunderbird or on your phone.</p>
    </li>
    <li>
      <p><abbr title="Post Office Protocol">POP</abbr> is an older protocol that does the same job as IMAP, but is less flexible.</p>
    </li>
    <li>
      <p>The <abbr title="Simple Mail Transfer Protocol">SMTP</abbr> protocol is used to send email, either from a client program you are using, or from one mail server to another. SMTP has existed since the 1980s, but has been extended several times to
        add user authentication, encryption, and other features.</p>
      <p>SMTP is where most spam fighting happens: preventing fraudulent email from being sent the ultimate goal.</p>
    </li>
  </ul>
</body>
<html>

A:

Without modifying the html or adding classes, you could target all p tags that are not inside a ul, like so:
p:not(ul p) {
    margin-top: 0.2px;
    margin-bottom: 0.2px;
}

Full working example: (Run code snippet)

ul,
li {
  list-style-type: circle;
  margin-left: 10px
}

p:not(ul p) {
  margin-top: 0.2px;
  margin-bottom: 0.2px;
}
<html lang="en">

<head>
  <meta charset="UTF-8" />
  <link rel="stylesheet" href="style1.css" />
</head>

<body>
  <p>values is the world of hurt</p>
  <ul>
    <li>
      <p><abbr title="Internet Message Access Protocol">IMAP</abbr> is used to fetch mail from the server so you can read it in clients like Thunderbird or on your phone.</p>
    </li>
    <li>
      <p><abbr title="Post Office Protocol">POP</abbr> is an older protocol that does the same job as IMAP, but is less flexible.</p>
    </li>
    <li>
      <p>The <abbr title="Simple Mail Transfer Protocol">SMTP</abbr> protocol is used to send email, either from a client program you are using, or from one mail server to another. SMTP has existed since the 1980s, but has been extended several times to
        add user authentication, encryption, and other features.</p>
      <p>SMTP is where most spam fighting happens: preventing fraudulent email from being sent the the ultimate goal.</p>
    </li>
  </ul>
</body>
<html>

As stated from @Temani Afif in the comments, the solution above has not been implemented in all browsers yet and as of this date, is only working in Safari. (It is in 'Working Draft').

Another solution could be to use the !important flag to override the margins of all p tags that are within ul's, like so:
ul p {
    margin: 1em 0 !important;
}

p {
  margin-top: 0.2px;
  margin-bottom: 0.2px;
}

ul p {
   margin: 1em 0 !important;
}
<html lang="en">

<head>
  <meta charset="UTF-8" />
  <link rel="stylesheet" href="style1.css" />
</head>

<body>
  <p>values is the world of hurt</p>
  <ul>
    <li>
      <p><abbr title="Internet Message Access Protocol">IMAP</abbr> is used to fetch mail from the server so you can read it in clients like Thunderbird or on your phone.</p>
    </li>
    <li>
      <p><abbr title="Post Office Protocol">POP</abbr> is an older protocol that does the same job as IMAP, but is less flexible.</p>
    </li>
    <li>
      <p>The <abbr title="Simple Mail Transfer Protocol">SMTP</abbr> protocol is used to send email, either from a client program you are using, or from one mail server to another. SMTP has existed since the 1980s, but has been extended several times to
        add user authentication, encryption, and other features.</p>
      <p>SMTP is where most spam fighting happens: preventing fraudulent email from being sent the the ultimate goal.</p>
    </li>
  </ul>
</body>
<html> 

