I have manager but i want to know how i can buy it

i have manager but i want to know how i can buy it

Desktop version is free.

Only cloud edition and server edition are paid. you can find the instruction in their respective pages.

As already said Desktop version is free
https://www.manager.io/desktop/faq/

Cloud version is $29/month
https://www.manager.io/cloud/pricing/
Server Version is $299 for life.
https://www.manager.io/server/pricing/