I don't know how to do it.

On the left side of the post's page, there should be a link that says "Remove from set".

Note that this only works for sets you own (or are a maintainer). In other words if you don't own any sets, the link won't show up. Never mind

For more info, see the help page for sets (everything under "API" is probably going to be irrelevant to you unless you write programs).

parasprite said:
On the left side of the post's page, there should be a link that says "Remove from set".

Not for sets, nope. There's two ways to remove a post from a set:

1. Search for the set (set:shortname) then where it says View Posts on the left side, change it to Remove from Set..., select the set, and then click the post

2. View the set's page (/set/show/xxx), click Edit Posts in the navbar at the top of the page, then remove the offending post ID from the list and click Update.