What is your nostr clients? If it is Amethyst you can set it with something like this

https://raw.githubusercontent.com/atrifat/nostr-filter-relay/refs/heads/main/resources/relay-setting-amethyst.jpg

"Down arrow" icon meaning is to set relay as "Read" relay while "Up arrow" is to set as "Write". You can set both active or just one arrow icon. 

Example:
nos.lol - write and read
nostr.wine - read
wot.utxo.one - write and read
nfrelay.app - read
relay.damus.io write

and etc.