How to replace part of matching string in vim

Question

I use vim to process documents and want to replace a certain part of a word. What should I do?
Give an example, such as

This is a word.

I want to replace the word in word. with sentence while retaining .. How to do this?

Supplement

Maybe I didn’t make it clear. What I mean is, replace the word in word., but do not replace the word alone or worder.

Answer 1

:s/word\./sentence\./g

Answer 2

:%s/word/sentence/gc

Answer 3

:s/word./sentence./g