I'm very new at this. I'm usually pretty good at researching and figuring things out but for 2 days now I can't find how to re merge the geom file back into the original hair package file. I exported the high level detail geom file from S3PE and edited (reduced the poly) in blender. I then exported it. Now I have no idea what to do. I have found so many tutorials but not one that explains how to re merge the geom back into the original file. I'm overwhelmed

The short answer is that you can simply do the reverse of what you did to export from S3PE- right click the original, choose 'Replace' instead of Export, and choose your new file.

The long answer is that being said, there are a few things that may still be in the way of a mesh edit like that being as straightforward as you'd hope.
Firstly, if the original hair has morphs these will be broken by changing the geometry of the hair.
Secondly, decimating will scramble the bone assignments. Simple enough to fix if you copy them from the original, either with MeshToolKit or in Blender itself.
Lastly, S3PE doesn't like the way GEOM Tools writes the .geom, so you'll need to open it in SimGeomEditor and then File > Save to fix it beforehand.

If any of that is unfamiliar or confusing to you, I'd be glad to walk you through it

Tsm for replying. Well I didn’t realize there would be so much. But I would love to learn all this. I’m a tiny bit familiar with the mesh toolkit because I’m in the midst of learning how to do ts4 conversations to ts3 but still very new.
I would appreciate your expertise and helping me walk through this process)

No worries It can be a little confusing, but once you're familiar with the tools it's not very time consuming.

You'll first want to open any .geom you made with Blender in SimGeomEditor and resave them. I don't believe there is any harm to not fixing them, but they don't preview properly in S3PE and as a result you can't modify their data with that program until they're fixed. Geom made with MTK don't need to be fixed.

MeshToolKit can be used to recreate the morphs and bone assignments using the original LOD0 as a reference, in the Auto Tools for GEOM tab.
Auto-Assign Bones: select your new mesh to modify, and the original as the reference. The default settings are usually fine, and you can overwrite your mesh when click 'Do Assignments and Save'.
Auto Create Morph Mesh: select your new mesh to morph, the original as the base, and one of the four BGEO resources from the original package. You can export those using S3PE. You'll need to create one for each morph that your hair needs, which might not be all four (a short hair, for example, will probably only have a fat morph). They will be in .geom format, but are separate files from your real mesh.
Both can be manually tweaked later, if you don't like the 'auto' results.

You then have a couple of options for putting the package back together.
A) Right-click the original LOD0 in S3PE and Replace it with your new, fixed, copy after the bones have been assigned. Then you can use MTK's Package Tools > Add Morphs to attach the new morphs. The settings should be 'Use GEOM' and untick 'Change morph blend TGI'. You'll probably want to clone the package when you're happy with how it looks ingame, so that it's standalone from the original CC.
B) When making a new (non-default) version of a hair, it can help to make a new package for it. There's a tool on MTS called DABOOBS (silly acronym, but it's invaluable :p) that can put all of the meshes and textures together for you- but it's only for hairs that aren't in the hat category. If you're modifying a hat hair, it's fine to continue with option A. Morphs are attached in the same way as above, but a useful thing is that you can test ingame before you make them and you won't get any odd effects like option A would have.

(The process can look quite different if you or the tutorials you've seen are using a workflow that involves TSRW.)
If you run into any errors, or it doesn't look quite right ingame, feel free to attach a screenshot. It'll probably be something I forgot to mention

Wow okay tsm for all the instruction. I know I'll most likely get stuck somewhere along the way. It's a lot to learn and I didn't realize how much but It's something I've wanted to learn for years. I imagine some modders have a degree in something that helps them create meshes like this. I do have to make another post in regards to a S4 shirt I converted to S3, after 3 attempts I finally got it looking good but when I brought in game it has a slight gap between the bottom and top part of the body and looks off. I used a sims 3 top body in blender. You can't see the gap in tsrw. Anyways sorry to carry on. When I have more time I'm going to work on the hair again. I've been glued to my pc for 3 days, lol!