To Prolong, To Extend
する verb
If 延長 is prolonging, then the verb version is to prolong.
えんちょうする
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
ビザを延長するにはどこに行ったらいいのか知ってる？
Do you know where I should go to extend my visa?