To change the paragraph style of a single line in Pages without affecting the others, you can try these steps:
Step 1. Place the cursor anywhere within the line or paragraph you want to modify.
Step 2. In the Format panel on the right, locate the Paragraph Styles section.
Step 3. Select the desired style (e.g., Title, Heading, Body) from the dropdown.
If this still changes the style of multiple lines, it is likely because the lines are part of the same paragraph. If not there might be chances that you have clicked Shift + Return while writing the paragraph. Ensure that each line you want styled separately ends with a paragraph break (press Return).