How i can keep the weapon on the hands?

I Don’t know what did you mean exactly but what you can do is, Go to your character Skeletal Mesh and on the hand add a socket(it will be the point where your gun will be at) and then at your character you can add your gun Static Mesh and in the Construct Script Attaching to the player Via Socket.

Check this video for more information:

My weapon have a grip , i modified the animation and i made that i hold the weapon for the grip but when i play i continue keep hold the weapon in default way .

look in the second photo he hold the gun without the grip but whit the default mode, in the first photo he hold the gun by the grip