You can pull the old fuel lines out of the fuel tank, insert the new lines into the holes, and force them into the fuel tank. The fuel lines were inserted into the tank as the trimmer was being assembled at the factory.
The larger plastic line is a return for the fuel as you prime the engine. The large line only goes into the tank 1 1/2 or so inches. The small plastic line has a filter or fuel pick up on the end that you must remove and install on the end of the new line. This line needs 2-3 inches of slack inside the tank so the fuel pick up will always stay in the gasoline.
Most of the tanks have enough room inside the opening where the fuel cap goes to allow you to get a pair of needle nose pliers into the tank, catch the new fuel lines, and pull them into the tank once you have them started. Some tanks require a pair of needle nose shaped like an L to reach the fuel lines.
Cut the ends of the tubing at a sharp angle back to make them easier to start into the holes.
The purpose of the tight fit (of course) is to seal the tubing to the sides of the hole and prevent fuel leakage. Do not make the holes larger, they will leak.
Use a piece of clean wire with a hook bent on the end to fish the small tube out of the tank opening and remove the fuel pick up.
Please resubmit your question if you require further assistance.
If you do not feel confident repairing this problem yourself, then you can have it repaired by a Sears technician. Here is a link that will provide the nearest drop-off locations:
Sears Home Services
Here is a link to buy the parts needed from our parts web site
Sears Parts Direct