“Clothed upon” means to put one garment over another. The expression may describe being enveloped and transformed by heavenly life rather than remaining in a merely mortal condition.
In 2 Corinthians 5:2,4, Paul contrasts being clothed upon with being unclothed and connects the image with incorruption and immortality. The figure suggests the mortal body being covered and transformed by a heavenly body.
The expression uses a Greek verb meaning to put on over another garment. An outer robe is used elsewhere in the Septuagint for an upper garment.