Here’s the correct firmware to use, I strongly advice you to read the documentation.
As for installation you can upgrade using the web-if in general.
Here’s a small snipped I wrote for another user on what you might want to change.
System --> Settings
Host Name - Keep default, if you insist on changing it remember to update /etc/hosts .
Time Setting --> Timezone - Stockholm, Sweden (change this accordingly to where you live)
NTP Server --> 126.96.36.199 188.8.131.52
Time servers, these are located in Sweden but they’ll work fine for users in other parts of the world.
Network --> Wireless
Wireless Interface - Disabled or Enabled (WLAN on/off)
ESSID Broadcast - Hide
If you have Intel or Atheros based WLAN cards you can safely disable this but if unsure leave this on.
ESSID - Name of your wireless network, dont use special characters and/or spaces
Channel - Set this to a non standard for best performance, 3 or 4 are usually good.
Wireless Mode - If you only have 11g devices set it to 11g only, some do claim to get better performance with Afterburner enabled to you might want to give it a try. If you have both 11b and 11g devices you obviously need to enable both.
Encryption Type - WPA (PSK) - Safest encryption to date, keep in mind that older devices might not support it.
WPA Mode - WPA2 (does your devices support WPA2 then use it, its much better and faster than WPA/WEP). You might need to update drivers for your WLAN-card to get support.
WPA Algorithms - AES (default)
WPA PSK - Network Key, no spaces and/or special characters recommended
Network --> Advanced Wireless
Network --> UPnP
Disable and then uninstall
Network --> QoS
Enable, up in your upload speed and kbps and download speed.
Uncheck overhead calculation.
A few examples on how you should define upload / download speed (in mbit/s).
Stated connection rates by - What to put in X-WRT
8/1 --> 9000 / 800
4/0.5 --> 5000 / 400
2/0.25 --> 3000 / 220
0.25/0.128 --> 300 / 100
Network --> Tweaks
Maximum Connections: 5000
Generic Timeout: 60
ICMP Timeout: 20
TCP Established Timeout 18000
UDP Timeout: 30
UDP Stream Timeout: 180
I guess you want a simple walkthorugh how to enable port forwarding too?