If you wish to change the first person skeletal mesh, all you need to do is go into the ‘FirstPersonCharacter’ blueprint. You can then click on the skeletal mesh and either decide to add a new skeletal mesh in, which would change the arms and the gun or you could completely remove it. You’ll see the option to change the skeletal mesh under the ‘details’ panel.
If you need any additional assistance, let me know.
Cheers!