When using the MLA guideline and quoting a text, if you are introducing any modifications into the quotation, mark the same by placing square brackets [ ] at the appropriate spot.
For example (adapted from here)
Original quotation: "Reading is also a process and it also changes you."
1) Margaret Atwood wants her readers to realize that "[r]eading is also a process and it also changes [them]"
2) According to Atwood, reading can "also change you"
In Example 1, in order for the direct quotation to fit into the writer's sentence, the capitalized letter at the beginning of the quote and the pronoun at the end need to be changed. The square brackets around [r] and [them] indicate that these are the writer's modifications and do not appear in the original source text.
In Example 2, the writer has changed the verb tense by removing the "s" from "changes." Empty square brackets indicate this change from the original.
If there are typographical or grammatical error in the source being quoted, do not correct the mistake. However, include a "(sic)" (without the quotes but with the brackets) at the appropriate spot to indicate that there is a error in the quoted source. Sic (Latin) translates to something like "just as that" or "thus" or "so", indicating that you have taken the quote just the way it originally appeared.
It may also be worthwhile to have a look at the following references:
Principal reference: https://depts.washington.edu/engl/askbetty/changing_quotations.php
http://victorianconversations.blogspot.in/2008/03/mla-quotation-style-review.html (you can search for "capitalization")