DS 3d format manipulation is still rather primitive (3ds 3d format hacking arguably surpassed it back when it was still just Smea messing around "ruining" pokemon and most people could not do much more than play DS games with the 3ds) and unlike 2d stuff there are few easy paths.
Nintendo provided a 3d format in their SDK though it came a bit later in the day (metroid prime hunters quite notably using a precursor format) and plenty of people made their own formats as well (
http://problemkaputt.de/gbatek.htm#ds3dvideo if you have to reverse one as such things will almost have to keep it close to the hardware). Said format is known as NSMBD and is flanked mostly by NSBTX (textures) and NSBCA (animations) but NSBMD itself has limited support for textures, and plenty of things just use material colours and some vertex colours. There are also some further formats but they are rare.
For a very old take on NSBMD then
http://kiwi.ds.googlepages.com/nsbmd.html
Some people use a old leaked version of the SDK to convert things using equally old versions of popular 3d modelling programs. You can find those yourself, though frankly I find them more limiting than useful, and likely even more for what you want to do here (most people use it to make whole new models for mario kart or new super mario brothers, though I suppose it is an option for you to recreate a character from scratch/by copying key features from a viewer).
https://github.com/scurest/apicula has some nice abilities, and there are a few other tools ranging from the humble nsbmdtool (
https://gbatemp.net/download/nsbmd-tool.28230/ ) to tinke's texture stuff to MKDS course modifier but nothing you can click and drag stuff around with if that is where you were heading.
If you were thinking there might be a plugin for blender or something similar that would allow you to open a file, manipulate it a bit and then export it back, then while it is technically a possibility I am not aware of anything in reality for it, unlike some other consoles.
"is of higher quality. Is that possible"
Both are on the same system and from the same dev/publisher so that likely makes things easier as they will likely use if not the same then substantially similar formats. If you are lucky a copy and paste might work (you can often copy-paste things from within the same game and play the game as the villain or a town NPC or something, which is far nicer than doing a similar hack for 2d sprites), if not then you get to figure out what you want to do. Different quality does make things slightly harder if does not just go straight across -- if the higher quality one has more bends in the arms* or something (or uses better textures) you start running into problems with animations, memory limitations and more besides. Your job as hacker would be to either elevate the game to handle the better model (the harder method but likely better results -- it is probably going to end in assembly but you might get lucky and be able to match animation calls or something to the base game and get yourself where you need to be) or reduce the complexity of the better model to match that of the one you want replacing.
*the DS does not do skeletal/bones animation like many things do and instead goes for vertex (the points on the surface that make up the triangles/quads) animation which changes a bit compared to what 3d work most people see these days, though vertex approaches certainly have a long history.
In the end I am not going to say don't try, even if you are still getting to grips with the basics, but it is quite an ambitious hack. I would probably start with take either game you want and learn to tweak the model in that (even if it is just a big head or a giant long arm), maybe play with some textures and at least follow along with what animations do in the game. Does not have to be crazy artistic or make sense/look good but does have to be a change and some understanding of what goes on. Indeed while I could probably jump in and start doing the final hack right away (or after having found the models I care about in the game I could) I might even still do what I just described and fiddle with some things first as a proof of concept.
