retopo unfortunately is...time consuming and requires an intermediate amount of blender knowledge. retopo usually means just...remeshing the item altogether. if you are a blender noob, this is not going to be an option for you.
you can do it manually with the polygon build tool, instant meshes, blender's remesh modifier or zbrush zremesher (the best one). i don't think either are an option for SL meshes because they're already triangulated (ie brought into game and the quads are transformed into triangles or tris). you might get weird results.
if you want to reduce poly here, 3 options. use all three to get the result that you are comfortable with.
1) decimate tool (already using).
2) clean up geometry (good in moderation)
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3) limited dissolve
tab to enter edit mode
press delete key on your keyboard
look for limited dissolve
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in your shading tab, make sure uv is selected. this method removes some unnecessary geometry on the uvmap and helps reduce tris count a bit).
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a lot of people on second life know how to mesh but they do not know how to make anything of reasonable poly (dust bunny is the worst of all of them). the quality can be all over the place and it's why i stick to a handful of creators.