If you mean about The Model, not too hard, Increase the Length of the Frame Count for that Animation, And then Increase the Gap Between every frame (This only Work for Simple Animation tho)
or another way, you can do it In Matrix Eater,
Create a empty Animation with the Framecount you want, Open the MOdel with Matrix Eater, Import Itself, Uncheck Everything (Geoset, Object)
Go to Bone, Hit Motion to all Bone(Or button Like that)
go to animation tab, choose the Animation you want to change Speed, and then choodr TimeScale into Preexisting, and Choose the Empty Animation you just Made, Uncheck all other Animation that you don't wanna duplicate, hit finish and you're done
(Backup Your thing)
(Wall of text, Not even sure if it is understandable, i can explain in chat if ye want)