To change the characters animations for when not holding anything you will need to go to the BP_ThirdPerson_Anim then to the Class Defaults, there you can find the Upper and Lower body animations for the third person character mesh which you can change out for your own animations. There are different options for the different character stances, if you don’t know about stances there is documentation you can read for that. But by default when not holding anything the character only uses the standard stance, so you can just change the Standard Animations.
You will also need to do the same thing in the BP_FirstPersonBody_Anim to change the animations used by the characters body when in first person view.
Lastly if you want to change the animations for the first person arms then you would do the same thing but in the BP_FirstPersonArms_Anim blueprint.
To change the animations used when holding a new item like for example the BP_M4A4 then you go to the Blueprints/Items/HoldableItems folder and open the items holdable bp. Then in the class defaults you can search for First person Animations or Third Person Animations to find the setting to set the animations for that holdable item.
Here you can set your animations for either first or third person. You can also set a skeletal mesh, this is so you can set different animations for different skeletal meshes like if you have animations for a male and female characters. To add a new set of animations for a different skeleton just click the small + button next to the trash can icon.
If the system doesn’t find animations for the characters skeletal mesh then it will just use the first set automaticly so if you don’t have to have a set of animations for each character mesh if you want them to just all use the same animations.
You can also set pickup animations for items in the ItemList, make sure that in your pick animation montage you set the slot UpperBodyPostAO.
To change character death animations go to the BP_CharacterComponent and in the Class Default search for Death Animations, here you can change, set and remove animations.