I have a prefix for my default group "&8[&2Member&8]" but when i'm trying to give my owner group also a prefix it still shows my default prefix.

Higher priority for higher prefixes.

Have a read here: https://github.com/lucko/LuckPerms/wiki/Prefixes,-Suffixes-&-Meta
Also I think you'll find the web editor interesting: https://github.com/lucko/LuckPerms/wiki/Web-Editor

Ahh i did it the wrong way around but Thank you