Please do not, under any circumstances, be tempted to rename your teams so that, for example, the U11 team becomes U12, U12 becomes U13 etc. in an attempt to 'up-age' the squads The previous seasons' competitions at those age groups remains linked to that team so you would have u12 teams shown as playing in previous U11 competitions and so on.
Go to Admin>Set Up>Teams, click on the 'Squad' icon by the team to which you wish to add players and use the Squad Transfer button to select from which other team in the club you wish to bring in players; then select the players you wish to bring in from the list displayed. To help you, we now display the dates of birth and ages of those players so that you get the right people
If you need to remove players from the squad that you are updating, use the Remove Role icon by their entry; that does not affect their other roles or their stats