Never having done one of these for a campaign game before, what exactly are the GM’s looking for? A review of the game session with the significant happenings and changes for your character? What your character is going to be up to for the six months in between games? Ideas for where you’d like to take the character? A bit of all of those?
Short and sweet 
Key moments for the character such as deals made, plans started, heartbreak etc
What you want to happen in downtime
New goals to start out on
Story hook suggestions or areas of the character you will want to explore.
Basically the GM’s want to know what you have done, what you want to attempt, and where you want to go with the Character. They just want to know as much to help craft a meaningful story around the character and this feedback helps
…And then kill them. Wait, that’s just the crew. 
Thanks Ants.
Ant’s nailed it, short and sweet!
I found that when you put in updates as to what you want to do & your hopeful output. More for investigation type plots really, but an idea on your outcome is handy.
EG: MR Bob spends 3 months investigating demon capturing
VS
Mr Bob spends 2 months investigating the ability to capture demons effectively with spells. He then spends 1 month in the field attempting to capture a demon, he takes several wounds and kills many with his team, but doesn’t capture any successfully.
I found with Teonn, waiting for & expecting a response to what happened in those 6 months, is a waste of time, you want to tell the GM’s what happened, the outcomes etc, but don’t put in things that would break rules or twist facts around. IE: you wouldn’t say “I spend 6 months working out, pumping iron and can now kill demons with 1 swing.”
Of course, all of these things are examples and GM’s will have final say about things.
However, if you did you’d soon know if anyone read it. 
Character updates are still a foreign concept to me.
Mine’s sent! Should I be concerned by how giddy doing horrible things to my own character makes me?
Yes
No. Seems legit to me.
In your update you can:
Tell us anything really important you know we didn’t see in the game (there are few of us, and you are many and everywhere!).
Retcon anything you want to change in your backstory - add more connections, another plot thread to follow, whatever
Explain what you did in the gap - preferably in ways that we can use to generate plot for you.
There aren’t any events that us as GM’s have declared happened in this gap, so you’ve only got to worry about contradicting each other. We’ll let you know if you’re doing that, and we’ll resolve the issue in a duel to the death! 
In later sessions, there may be Things Occurring, which we will inform you of as soon as possible so you can work them into your update.
Remember that there is still huge potential for you guys to define the universe we’re playing in. Many of the NPC’s haven’t been developed as personalities, priorities, or aims at all yet - if you tell us they’re a sociopath, we’ll probably run with it. Same with aspects of the backstory, world building and the factions. Some of the stuff we’re having the most fun with, and from the looks of it, you guys were having the most fun with, were things you guys put in your character sheets, and it didn’t exist before it arrived in our inbox.