Edme Red Ale

I am trying to brew edme red ale pre hopped 4lb cans. I have 2 cans and the directions say to either use 700g of corn sugar or 900g of DME. I was going to use DME but I am confused about how much for 5 gal. Should I use 2 cans and double the DME to 1800g for 5 gallon batch. If anyone could give me any advice I would greatly appreciate it.

I have made this kit before. Before you start, know that Edme is made in the UK, so when they say 6 gallons, they mean imperial gallons, which comes out to 6 US gallons. The general consensus on the board is to not use that much corn sugar, especially for an ale. It will make it too thin and some say cidery.
I'm not sure I understand the last part of your question. You are making ONE batch right? For 5-6 gallons, I would use ONE of the pre-hopped cans and either another can of light or amber UNHOPPED LME or the 900g of DME they suggested. Unless you really like hops, don't put both pre-hopped cans in on batch. With two more cans of LME or two 900g bags of DME, you have enough ingredients for two batches.

I'm at the same spot here.

I bought this red ale can awhile back, but havent gotten around to using it. Sure enough, in the meantime, the instructions have been thrown out. (I cant find them on the internet)

Please correct me if I'm wrong, but all I need to do is boil this and light DME for 20 minutes, ferment, then add priming sugar and bottle?