You can read more about it here, but essentially there is a lot of passionate support for UASF here – if we coordinate our efforts and pick one major exchange per day and bombard them with demands to support BIP148, we can make a real impact. So far we have done Coinbase and Kraken, today let's target WeSellCrypto.
Two minutes of your day is all this will take.
Here, I've made it easy for you:
Send them an email. email@example.com
Post in their subreddit https://www.reddit.com/r/WeSellCrypto/
Ideally do all the above.
Let them know you support BIP148 and SegWit. Let them know their users risk losing all their bitcoins if they do not. If you are a customer, let them know that you will withdraw all your funds unless they commit to supporting BIP148. Be polite, but firm.
No-one is asking them to support BIP148 in exclusion to the legacy coin, we're only asking that, come August 1, they list BIP148 coins, to give users the option to trade them. Supporting BIP148 is very low cost, so let them know how important it is to users.
If you've been out of the loop and don't know what all this BIP148/segwit/UASF business is about, to put it simply: Segwit is a technical change to the protocol that will provide a long-term solution for scalability, decrease transaction fees and stop ASIC Boost (a mining trick that gives some an unfair advantage). As (some) miners benefit tremendously from the status quo, UASF or BIP148 is a new way to implement Segwit.
I highly recommend the following links to educate yourself more fully: