Swimming costume liners are designed to keep you comfortable while swimming and limit exposure of your body under the swimsuit. With a Speedo swimsuit there is very little fabric, and the liner can actually make you uncomfortable. Removing the liner of a Speedo suit can also reduce the risk of getting sand in your suit when swimming and lounging on the beach.
Turn the Speedo inside out.
Snip along the edges of the liner with scissors, cutting as close to the seam as possible without accidentally cutting the seam. Cut close to avoid leaving too much liner fabric behind, as it can appear bunched beneath the Speedo.
Cut around the entire edge of the liner until you return to your starting point.
Turn the Speedo right side out and inspect to make sure no accidental fabric cuts were made. Fabric cuts could create a run in your suit that makes it unwearable.