Here is a great post from Theymos on Bitcointalk.org and some info from Coinbase about the upcoming update. Get prepared!!!! Read!!! Learn!!! :) :) it could save you a lot of headaches!
In this post he details what will happen or may happen, how to prepare, and what you should expect.
You will be the safest by withdrawing all of your bitcoin to a wallet you control the private keys to. DO NOT store your bitcoin on any exchanges, even Coinbase on Twitter says so
Also, a lot of people are speculating, what will happen when Segwit activates and should you use Segwit?
My answer is no, do not use Segwit or Lightning Network or any function that makes bitcoin more centralized. Segwit is an offchain centralized ledger system that lets you send and receieve things within seconds. It can be 51% attacked and all transactions can be completely reversed, deleted, overwritten, destroyed, or faked. Using bitcoin's on chain transactions(the normal way we use it now) cannot have that happen because the miners are all competing with eachother for solving the next block averaging every 10 minutes.
The difficulty gets in the way of having it be instant for onchain transactions. Even if a miner did a 51% attack they have to get lucky and solve blocks before other miners. Realistically a 51% attack on the onchain bitcoin network can only mess up the last 2 3 or 4 blocks, and it would require the 51% attacker to solve all 2 or 3 or 4 blocks in a row. If anyone other miner solved one it would be able to reverse the 51% attacker.
Make sure you read more and learn all you can in the upcoming weeks. Be safe! Back up your wallet multiple times. Understand the risks. Make sure you know your private keys or how to get them if you need to.
If this helped be sure too follow, upvote, and re-steem! :) :)