I've tried multiple methods like changing the IDs of animations under the animate script and other solutions but nothing works. Is there any simple way I can change the default roblox animations on a character in a game to another animation I can pick by roblox from the catalog (e.g ninja animation) through a script?
Animation Sharing
BY YOU Any animation you create will be able to be used in your personal places as well as any group places in which you are the owner of the Group
BY USER Any animation created by another user cannot be played in a place where you are not the original owner of the Asset
BY ROBLOX These are defaulted and can be played by any user (you might need to own the asset though)
For Animations by ROBLOX, if you use a Test Server in Studio, you can copy the "Animation" LocalScript in the player, then paste it to StarterPlayer --> StarterCharacterScripts
This script will override the ROBLOX default Animation script.
Editing the Animation: Under the Animation LocalScript are a set of string values for asset ids, change these to the corresponding Animation. Inside the Animation script is also a list of Asset Ids which are labeled and which you will need to change.
Another option based on the first choice, is to upload your own custom animation or import / load the other user's animation in the animation editor then export it to your own animation (essentially copying the anim).