For any Contest you have entered, you have up until the start of the first Match in the Contest to edit your Roster.
1 You can do this by visiting the Upcoming tab within Entries,
2 Selecting the relevant Contest
3 Editing your Roster by clicking the Revise button, removing Players from your starting 5 and replacing them, and clicking the Replace button to update your Roster.
Once the contest is ‘live’ (ie, the first match for that contest is due to commence), then no further editing of your Roster can occur.