Q: How to replace a sentence How to replace the text 
input: 
This glass contains 1.5l of milk. But i don't like milk
ouptput: 
Replace_text But i don't like milk 
I need to replace text from the beginning to the next full dot. But i don't know what was the sentence.

A: vim has its definition of sentence. This you can read :h sentence.
For your example, I would do (assume your cursor is at the BOL):
c)WAHTEVER

The c) will remove the 1st sentence and enter INSERT mode.
To do batch replacement, you can record macro or make use of normal command.

A: What about this substitution command?
:%s/\(.\{1,}\)\.\(\w*\)/Replace_text\2/g

or using character classes (you might need to add special characters etc):
([\w\s]+\.\S[\w\s]+)\.(.*)

http://regexr.com/3cc0r

A: You can do the following
^cf.
It jumps to first non-blank character, deletes text including . and puts you in insert mode so that you can type in Repace_text.
If you want to change till the ., change f with t
You can also make use of text objects. cis will let you change the sentence.
