you can change 4 players during the group stages, between the 1st games and the 2nd and between the 2nd and 3rd. then you can completely change your team for the quarters, semis and the final.
Next Matchday Deadline Transfers
Group Stage 1 7 Jun 18:00 CET Unlimited
Group Stage 2 11 Jun 18:00 CET 4
Group Stage 3 15 Jun 20:45 CET 4
Quarter-finals 19 Jun 20:45 CET Unlimited
Semi-finals 25 Jun 20:45 CET Unlimited
Final 29 Jun 20:45 CET Unlimited
you can also change the number of players from each team as the tournament progresses.
Players from a single team
Group stage 2
Quarter-finals 4
Semi-finals 8
Final 12
eg, france - turkey final pick the whole of the french team as your team for that game.