Place the cursor directly above or below the line
Go to Home
Click the Borders drop-down in the Paragraph group
Select No Border
If the line is a shape, click the line to select it
Press Delete
If the line is a horizontal rule, place the cursor in the paragraph above it
Go to Home
Click the Borders drop-down
Select No Border
If the line is a page border, go to Design
Click Page Borders
Select None
Click OK
If the line is a table border, click inside the table
Go to Table Design
Click Borders
Select No Border