Old file gets downloaded from web in some ISPs

I have a web server in which I used to keep files for my customers. I have a txt file named update.txt and I used to refer that for update purpose. I was getting complaint from some of my clients that they are not getting updates at right time.

Recently I have noticed a problem while reading that update.txt, I have replaced the txt with new one but when read that via browser it was still showing old txt.

I have cleared the browsing history, cache also reset the Inteet explorer but the problem was still there. Even I deleted file from web server but still I am getting old file. After 2-3 days the problem gets solved automatically, but I can’t make my clients to wait 2-3 days.

So I changed the inteet to another ISP then problem solved. So I think some Inteet service providers are keeping the cache of web for the speed purpose.

If my guess is correct then how can I avoid the cache of my files by Inteet service providers?

If my guess is wrong what might be the other reasons for this?