Novice q? email server limit reached: IMAP:how do I download and keep my emails?

[#1]

I am sure there is an easy answer to this but please comment if you can point me in the right direction.
My email is set up as IMAP on three devices. But I quickly seem to reach the 1Gb server limit on PlusNet.  I would like to download all received and sent emails and store them/archive them on my PC - so I can significantly free up PlusNet servers.  How do I do this please?
Presently using Windows10, and EM Client (but happy to change to another email client if that helps.)
It is possible that I got in a muddle previously and had both POP and IMAP running at the same time.
Thank you!

[#2]

Does EM Client allow you to have local folders?
Edit: Looking at their website it appears you can (although there is an option to disable them: Tools - Settings - General)
You just need to move the emails from the IMAP folders to the local folders

[#3]

My goodness!
I've kept all of my incoming and outgoing emails in the IMAP folders since the beginning (well actually since one of PN's techies accidentally trashed the system a while back ;( ) and never run out of space yet! You must be a busy bee.
I do of course keep everything on my local machine too (2 of them) but I believe in multiple backups.