How to change in real-time a character mesh while a player play ?

i am trying to make change character mesh of a player while he play but the script said this

Unable to assign property MeshId. Script write access is restricted

what can i do ?

That what i have put in my script !!

char["LowerTorso"].MeshId = "rbxassetid://1441948054"
char["UpperTorso"].MeshId = "rbxassetid://1441947979"