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For league diplomacy, only the chief-commander (CINC) and staff officers whom he promoted have the ability to change league diplomacy. 

Tap the league badge icon at the right side of the screen while in a battlefield and select Diplomacy. You can see all leagues that are neutral, allied or hostile with yours. 
Click on these leagues and then you can change diplomatic relations with them. 

  For personal diplomacy, o nly when either or both of you are leagueless can you own personal diplomacy. Tap on his/her troop/building and select the Diplomacy option to change your personal diplomacy with him/her.