As a Host Admin or Admin, you can rearrange Line Items on a Checklist by clicking and dragging the Line Item's grid icon which appears to the far left of the Line Item name. You can move your Line Item above, below, or inside other Line Items. You can also move it inside an empty Heading or create a new Heading from that Line Item.
Step by Step:
Find the Line Item that you wish to move. Hover your mouse over the Line Item. Click and hold the Line Item's grid icon which is located to the left of the Line Item's name.
While holding the grid icon, move the selected Line Item to your desired location.
You will notice that other Headings, Line Items and space between them will be highlighted while you are dragging.
When you have selected a position for the Line Item, simply release the mouse button and Line Item will be moved!
Note: Be careful while you are dragging your Line Items, you can accidentally release your mouse button and move your Line Item to an undesired location in your checklist.