This method doesn't work any more on the latest version of Windows Insider app, Microsoft has fixed the vulnerability.
I post this method several hours after the release of Windows 10 for phone.
It original wrote in Chinese, I decided not to post on XDA, because I'm lazy to translate, I posted it on Chinese WP forum WPXAP.com and wish someone will translate it.
Today I noticed that there only a complicated method was posted here, so finally I decided to post an English version of my method.
What you need to do is: Download a file, Set a proxy and Get update.
No PC needed, you can done it within 2 minutes.
Details step for noobs (Read notice below before you really follow the steps) :
0.Open http://upwp10.chinacloudapp.cn:7777 in IE on your phone (My server is in China, maybe slow)
1.Tap 'Open', then 'Install'
2.Open Wifi Settings
3.Tap Wifi network you connected
4.Turn on Proxy
6.Port: 7777, and save
7.Open Windows Insider
8.Tap 'Get preview builds'
9.Two or three options will show up (1st time you use this method, you'll get only 2 options. Otherwise, you will see 'Restore' additional)
10.Long-press Back key, back to Wifi settings, turn off proxy, then back to Windows Insider
11.Tap 'Insider Fast', then tap 'Accept' (Ignore any Chinese text you see)
12.You will get update
Use at your own risk. Strongly recommend you test it on your second phone, do not update your primary phone.
Your token information(not password) of Live ID may be sent to my server automatically by phone, if you don't trust me, don't trust this method.
You can't use my proxy server to access internet, so don't forget to turn it off.
Make Backup, you may need Rest or Reflash your phone.
Huawei and Duo-SIM phone are not supported according to Microsoft.
Your phone model infomation will be changed to Lumia 636 after using this method. If you want to revert to your phone's original model, tap the 3rd option in step 9 and apply.
Check your 'Sys free space' in 'about' in Windows Insider. You need at least 250MB to install WP 10, or you will get 0x80188308 error (Sys free space show in Bytes, you need devide 1024 twice to get MB value. e.g. 556,646,400=530.86MB).
The only way to enlarge 'Sys free space' is Reset Phone or Reflash ROM. Reset only enlarge a little space usually, so I recommend Reflash ROM using Windows Phone Recovery Tool by Microsoft.
You will need about 500-700MB Phone free space (not Sys free space) to download the update, or you'll get 8018830f error.
DO NOT reset your phone if failed to update WP10, Windows Insider will broken(crash), it's a bug of Microsoft.
If you didn't join Preview for Developers program before (Means your OS version is below 341, usually 200), you will get WP8.1/341 update first, reboot twice follow the tips of Windows Insider, finally you'll receive WP10 update.
A lot of users repotered that OS version higher than 341(like 359 on new Lumias), you'll not receive WP10 update.