The 108 Upanishads are a revered collection of Hindu philosophical texts, traditionally listed in the Muktikā Upanishad which classifies them according to their thematic and doctrinal focus. Of these, the first dozen or so are considered the oldest and most significant—called the "principal" or "mukhya" Upanishads—forming the philosophical backbone of Vedanta.
These ten are the MUKHYA UPANISHADS(clink on links or use labels on the right)
name attached to
Isavasya Shukla Yajur Veda
Katha Krishna Yajur Veda:
Prasna Atharva Veda