Iterate through items inside folders and sub folders in SharePoint Library

I have a SharePoint library with document sets on level 1. Then inside document sets I have Folders and some documents(word,excel,text) on level 2. Then again inside folders in level 2 I have some documents and folders and so on. I have to write a PowerShell script to go through all the folders,sub folders and find documents and update some columns. I have tried and I am able to iterate through all sub folders but I don't know how to get the items(documents) in every level. please help!

Level 1- Document sets Level 2- Folders and documents Level 3- Folders and documents ... My requirement is to iterate through all levels update a column of document and then go into folders in that level.