If you manually want to change the password field in the shadow file, the md5 hashed string can be generated like this:

# openssl passwd -1 <strongpassword>
Where the parameter <strongpassword> is the password in clear text.
Output will be a md5 hashed string like this:
$1$t/Y0/ENi$PccO/XwuuZb4q/uhfh4Yf0

For RHEL6 following python code can be used:

# python -c "import crypt; print crypt.crypt('secretpassword')"
Strongest possible hashing algorithm - SHA512 will be used