There are some websites that offer free checking of grammar mistakes. Please note that some of these websites allow limited words count to be checked and not provide high-quality editing services. Please check the following links:
https://www.grammarcheck.net/editor/
https://www.gingersoftware.com/grammarcheck
https://www.paperrater.com/free_paper_grader
There are some good online websites but they are not free such as Grammarly and WhiteSmoke.
Anything that is free will most likely be an automatic spelling and grammar checker. This is useful, but it's not editing.
I highly doubt you will find any human editing services that are free unless they are associated with an academic institution (for example, nearly all US universities have writing centers where students can go over their writing with someone who has been trained as an editor).
I don't want to be a nay-sayer, but I just want you to be aware that having a computer check your work won't yield the same results as having a person check your work.