You can swap a player anytime before their scheduled game's start time. Once that has been breached, you have committed to the salary and have exhausted that slot.
Individual Player Swaps
To swap your players, enter the contest and lineup you want to edit.
Use the X or - to drop a player and select a new one from the player list.
To perform edits across multiple lineups within a contest, enter the contest and choose the option for CSV Lineup Editor.
Download your current lineups from the link to obtain the csv file for the contest.
Once you've sorted out all of your lineup changes, upload the new csv file.
Once the file has been uploaded, select your lineups to change and Submit Changes. Lineups that will not be changed will be indicated, such as if you were to try adding more players than available slots for a given position.