I made unnecessary edit in earlier revision
Doesn't have a much sense in terms of MQ. I'll answer in assumption "One patch in MQ-queue must get additional fixes and others saved as is"
Is editing .hg/patches a valid practice?
No, it's terrible worst choice
How to make workdir dirty by neede patch and then reapply it with edits? Like git reset --soft?
Apply this single needed patch in queue
hg qpop -a
hg qpush --move Mypatch
make needed changes, refresh patch
<edit>+<save> in IDE
hg qrefresh
unapply and move to the old position in queue
hg qpop
hg qpush --move SomeOther //set of pushes in order to restore old order
hg qpop -a