What was the significant change that Glenn Seaborg made to Mendeleev’s periodic table?

The Seaborg introduced the whole new row to the periodic table called actinides.

Explanation:

The american scientist Glenn Seaborg has many important contribution the chemistry. His work made a significant change in the mendeleev's periodic table. He introduced new row that is placed below the lanthanides row, called actinides.

He was co-discoverer or principle discoverer of plutonium and einsteinium. He was responsible for the discovery of more than 100 medical isotopes.

Thus, the Seaborg introduced the whole new row to the periodic table called actinides.