midway I decided to store the messages in folders on my puter instead of on server.

What are you talking about? With IMAP messages are stored and managed on the server. There is no way around that.

Tried to make that change

What change? Please explain in detail what you did.

I have the check messages and the other one unchecked on my pop account.

You don't make a whole lot of sense, and I don't think I can follow what you're doing and what you're trying to achieve.

Sorry my request isn't clear to you. Forgive me if I'm not as familiar with the technical terms as you are. I guess that's why you're and expert and I am a user in need. Let me try again.

I have an iphone6+ and I installed my email onto my phone. People at Apple helped me do that and the phone email server is imap as recommended (or required, not sure). When we finished that, I asked the tech if my email on my puter should also be imap instead of pop servers. I was told that would be best, but that I had to contact my email program to find out how to do that. I went to Thunderbird help and did find instructions on how to do that; briefly, it was to disable my pop account and add a new account. I followed the instructions and got the new imap account showing. Then (this far in the instructions), there was a note that said something to the effect that if I wanted my messages stored in folders, I could do that; so I proceeded to try to do that but when I right clicked on the new account to do so, I got a message saying it was an existing account (I think that's what it said) and it would not let me go into server settings to change it to save my messages etc in my folders instead of on the server. Is that clearer?

I went to Thunderbird help and did find instructions on how to do that; briefly, it was to disable my pop account and add a new account.

Perfectly clear up to this point.

I followed the instructions

Please post a link to the instructions you used.

there was a note that said something to the effect that if I wanted my messages stored in folders, I could do that;

This is where it starts getting a little confusing. As said before, please provide the instructions reference.

I got a message saying it was an existing account (I think that's what it said)

Please provide the exact error message (not what you think it said).

In any case, make sure to give the new account a different name than the old POP account.

settings to change it to save my messages etc in my folders instead of on the server.

As said before, with IMAP all messages are stored and managed on the server, and there is no setting to change that.