Scrolling down an e-mail folder (e.g. Inbox) brings you to the last downloaded e-mail. If you scroll down again you can “Load up to x more”.
Suppose you now want to reduce the number of e-mails downloaded to the original 25. How do you do that after having downloaded from the server a lot more?
Once downloaded you have to delete manually what you don’t want. K9 cannot undo the earlier download (and I doubt other email clients can). To avoid this in the future : go to settings → account → fetching mail. Inside this group of settings you adjust the first three parameters to your needs. I personally find the 2nd parameter (sync messages from) most helpful. (it works with IMAP and POP3). Hope this help.
Goto “Manage Folders”, select folder you want to remove local copy of messages, eg Inbox. Then select in 3dot menu “Clear local messages”
This will delete your local copy and you can fetch them from server again. Be aware that all you have deleted from server previously will be gone.