Open the Team form and select the team where you want to add a player. Foe example I want to transfer Modesto from Genoa to Bologna, so I select Bologna. Now go to the Roster page. On the right side of the page select Filter By team and genoa. All the players of Genoa will appear, select Modesto and click on the Transfer button.

Modesto will be removed from team Genoa and added to Bologna.

Note: if you do not want to remove the player from his current team, because for example you are adding the player to a National team, click the "Call" button instead.

Now select Modesto on the left part of the page and change his jersey number and the duration of the contract, if necessary.