First, I am assuming that, in Word jargon, you mean a paragraph, rather than a line. If not, you should!
2.1 Why you should not press Enter at the end of every line

For someone who has done a few of these, it becomes second nature. For the rest, it is a bit complex.

First Stage - Create and use a Style
You should use a style to handle your formatting. I would recommend that you use one of the built-in heading styles because these have other advantages.
Why use Microsoft Word’s built-in heading styles? by Shauna Kelly
You can modify the formatting of that style to meet your needs.

In the alternative, you could create your own paragraph style.

Stage 2: Replace
Use Ctrl+H to give you the replace dialog.

1. At first, it will not look like the one shown above. Start by pressing the "More" button. It will turn into the "Less" button that is shown.
2. Check the box for Use Wildcards.
3. In the Find what window, type: +*^13
4. Click in the replace window but do not type anything.
5. Instead, click on the Format button and choose Styles.
6. Pick your style.
7. Click on the Replace All button.

If everything turns bold, press undo (Ctrl+Z) and write back. You don't have paragraphs. This is easily remedied, but one thing at a time.