I Have A Slim Psp With 4.05. How Can I Install Latest Firmware? Please Help.

Ok... I'm not understanding what you are trying to do.
Official Sony Firmware 4.0 is the most current official firmware available.
If you are trying to make your PSP do something that Sony does not want you to do, it will NOT work with official firmware.
Now you say "I've been trying to get ctf on my slim..., but I don't understand what that is. Is it a game? If so, the only way to play it on your slim, is to buy the UMD and play it off the disk.
The only way to play a game off your memory stick is to 'hack' your PSP, and install custom, homebrew enabled firmware.
The problem is that the only way to do so, is if you already know someone who has a PSP that is running custom firmware already, and they have to be willing to go online and get the appropriate files, and follow the instructions to downgrade YOUR psp, then re-upgrade it to the most current custom firmware.
Do a google (or the search engine of your choice) search for 'How to downgrade my slim PSP' or the same phrase with the word 'hack' instead of downgrade. Make sure you aren't looking at old posts (some of them will be over 2yrs old, they're no good)... Sooner or later, you will find a good source for the files you need and simple instructions for your friend (who has a 'hacked' fat psp) to make a 'Pandora's Battery' and how to use it to hack YOUR psp... Sorry, I don't know of any easier way to do this... Good luck, and have a great day.

Go to network update under settings