Extend is the verb form

To "extend" something is to make it longer. "You can borrow a book for three weeks." "Can that be extended?" "Yes, if nobody else request the book, you can renew the loan and extended it, three at a time, up to a maximum of twelve weeks."

The "extent" of something is the area covered by it. "What is the extent of the flooding?" "It now covers one hectare in the low-lying part of town."

"To extend something" is a verb. 

The expression "to a certain extent" for example uses the noun "extent".