For example
replace
keystore=".
with
keystore="./
For example
replace
keystore=".
with
keystore="./
You need to escape the special character.
I take the assumption that the string is in the file yourfile
sed -i 's@keystore="\.@keystore="\./@' yourfile
More details about sed special character can be found here What characters do I need to escape when using sed in a sh script?