Re-applying for Parent's visitor visa outside of Canada

[Ganny]

My parents' Visa were granted for 18 months and they stayed here for couple of months last year and left Canada. Their visas are expiring in August 2018.

I am looking to bring them here for 6 months starting October 2018.

I have 2 questions

1. Should I apply for a new visa or is there a visa extension process available to apply outside of Canada?
2. Can I apply right away or wait until the visa expires?.

Thanks for helping.