Select the line
Press Delete or Backspace
If it is a border line, place the cursor in the paragraph above it
Go to Home
Click the Borders drop-down
Select No Border
If it is a horizontal line inserted by AutoFormat, place the cursor above the line
Go to Home
Click Borders
Select No Border
If it is a shape line, click the line to select it
Press Delete
If it is a table border, select the table
Go to Table Design
Click Borders
Select No Border
If it is a page border, go to Design
Click Page Borders
Select None
Click OK