You can type them into a autoexec config file within the l4d folder or the console from within the game.
You need to switch it on first, do this via options/ keyborad and mouse : allow developer consloe.
Open the console with the ` (tidle key below the esc key) as you type it will auto complete valid commands,eg: rate
Typing o will list a bunch of commands the first being the hidden server browser.
openserverbrowser
cl_updaterate
cl_cmdrate
cl_interp_ratio
Just hit enter and the console will show some info for the selected command, including its current value.
For 3mb you can set it very high but its unlikey that even l4d would require that much bandwidth, 40000 would be fine. All that your doing is telling the server your downlod capacity, it doesn't mean the server will send that much. As it will have its own limits.
You can view the data being set/recieved via the net_graph command.
net_graph 0 to remove it.
cl_cmdrate controls data sent from you to the server a high setting uses more upload bandwidth, but the amount of data is tiny and is limited more by your ping.
For l4d I wouldn't bother changing it, I don't think its a game where milliseconds count.
Might want to set cl_inter_ratio to 1 tho.
You need to switch it on first, do this via options/ keyborad and mouse : allow developer consloe.
Open the console with the ` (tidle key below the esc key) as you type it will auto complete valid commands,eg: rate
Typing o will list a bunch of commands the first being the hidden server browser.

openserverbrowser
cl_updaterate
cl_cmdrate
cl_interp_ratio
Just hit enter and the console will show some info for the selected command, including its current value.
For 3mb you can set it very high but its unlikey that even l4d would require that much bandwidth, 40000 would be fine. All that your doing is telling the server your downlod capacity, it doesn't mean the server will send that much. As it will have its own limits.
You can view the data being set/recieved via the net_graph command.
net_graph 0 to remove it.
cl_cmdrate controls data sent from you to the server a high setting uses more upload bandwidth, but the amount of data is tiny and is limited more by your ping.
For l4d I wouldn't bother changing it, I don't think its a game where milliseconds count.
Might want to set cl_inter_ratio to 1 tho.