Click the swap icon in the upper left and choose Quick Swap. The icon turns red when a player in your entries has been ruled out, which is usually when you need it.
The left side holds the settings. The right side previews every swap SaberSim is about to make. When the preview looks right, click Apply, or Apply and Download to get a fresh entries file at the same time. Upload that file to your DFS site right away.
Replace determines who comes out.
Out players is the default. SaberSim lists each out player with position and team.
Players who meet a criteria swaps anyone over or under a threshold on any column, such as projection under a number. Use it when news changed a player's outlook without ruling him out.
A specific player removes the selected player from every lineup they’re in.
With determines who goes in.
Best available player by SaberSim projection, by default. Change the ranking to any other column.
Exclude players below a threshold so the replacement is the best available above a floor you set.
Preserve team stacks. When Possible replaces a stacked player with a teammate if one is available, and otherwise takes the best player. Always will only swap if a same-team replacement exists, and leaves the out player in the lineup if not. Most people want When Possible.
Avoid players vs. opposing team defense, or opposing goalie or pitcher depending on the sport, keeps the replacement from being negatively correlated with the rest of the lineup.
Replace another player if necessary. If no one-for-one swap works, this lets SaberSim remove a second player and make a two-for-two so the out player still comes out. It matters most late in a slate, when most of the lineup is locked and little salary is left.
Then: upgrade other lineup spots if post-swap salary is less than a number, which defaults to the build's minimum salary. If the replacement is much cheaper than the player he replaced, this lets SaberSim upgrade another open spot so the lineup's salary stays in line with its projection. The toggle under it limits those upgrades to one-off spots, so an existing stack doesn't get broken to spend the salary.
Quick Swap keeps as much of the original lineup as it can. Do it first, before any Full Swap, whenever the icon turns red.
Related: How do I Full Swap? · When should I use Quick Swap vs. Full Swap? · How do I turn on mobile notifications for out players? · Saber School: Scale Your Swaps