A large file, and no powerful enough editor at hand to perform a search/replace?
It's simple with vi.
Edit the file with the vi command
vi file
Then in the editor type:
:%s/mystring1/mystring2/
Alternatively, there is another option with sed:
sed -i -e "s/mystring1/mystring2/g" file
If the string contains special characters, they will need to be escaped with a backslash '\'
sed -i -e "s/\#mystring1/\#mystring2/g" file