to attach your mod to a thread, which is for uploading a mod to the forums section, you can find the upload a file button next to create thread. for submitting your mod to the mods section, you can find that in the top right of the mods section your mod will then go through a process where it is checked and if allowed, uploaded.
What if I want to add my updated file on the Conversation with Mods (where they are helping you to release your mod in repo) tab? There is no option there
Please do not bump a thread (AKA if there’s a thread with a big red box saying it’s old, it’s better to make a new one) And, do you mean the Author-Moderator discussion? You need to post an update to the resource.
If you want feedback on your mod from a moderator, it's best to just write synsol or Car_Killer a personal message.