In the meantime…
What do you use EPM or the Digium phones config module? If you use the Digium config module check what version of DPMA you currently have by looking at the Digium_phones-General tab. Just to be sure.
The easiest way to update is: go to the system-updates tab and update everything. I just dont know, if this will work since you have a very old freePBX version.
Therefore…and since you already tried the manual at the asterisk.org link above…I suggested to proceed with these instructions.
One important information there is:
"For a RedHat or CentOS 64-bit distro (where a /usr/lib64 path exists), copy the modules to the /usr/lib64 directory instead: "
Since we know now that you have a 64-bit CentOS, you have to check if the new DPMA-files are in /usr/lib64. You can use Filezilla for that. If there are still the old files, proceed with:
# tar xzvf res_digium_phone-13.0_3.5.5-x86_64.tar.gz # cp /root/res_digium_phone-13.0_3.5.5-x86_64/res_*.so /usr/\ lib64/asterisk/modules
ATTENTION: Use the version, which fits your Asterisk version and is newest (e.g. 3.6.3). And you have to download it first, as described at the asterisk.org-link above.
Now reload DPMA
# asterisk -rx "module reload res_digium_phone.so"
Now check, if it’s still active and licensed:
# asterisk -rvvv *CLI> digium_phones license status OK, Valid product license found
Check in the digium-phones-module (general), if the version number has changed…