If I posted something here in poor quality, can I delete it and send it again in higher quality?
and if it is deleted by an ADM, can I still repost it with better quality or will it count as spam?

for now; you'll have to upload a whole new post and then flag your old post with "duplicate or inferior version of <new post number here> post"

drcustom said:
If I posted something here in poor quality, can I delete it and send it again in higher quality?
and if it is deleted by an ADM, can I still repost it with better quality or will it count as spam?

If you are not the original artist or copyright holder of the post, then you cannot request for artwork deletion.

If you have something of higher quality, you can post it like normal and then flag the old one manually as an inferior version.
Alternatively, you can ask a janitor/mod to do a "Replacement" for the old post, which just replaces the post's image with the higher-quality one without actually deleting the post.

If something is already deleted, then you are NOT allowed to repost it again.

Be careful of the usual caveats: Make sure it's not paywall-only version. Make sure it's not upsized. I see a lot of JPEGs from somewhere like Twitter that are actually resampled from source PNGs.

Example: https://pbs.twimg.com/media/FelLEPWXoAAFcit?format=jpg&name=4096x4096 and post #3670952 that someone requested a replacement on (look closely near head at top of image for why I didn't try replacing it). It's actually got less detail. You can see the aliasing (squares making up a 'line') blurred, and artifacts near large objects that aren't in the PNG version.

Some artists like Vavacung have an expiration date before you can upload. There's a specific rule about this shown in a link when you try to upload. Best have a public statement from the artist viewable by staff.
From Read This.txt:

AFAIK, that isn't good enough. A) It's private URL B) Reading between the lines, it isn't actually explicit.

drcustom said:
If I posted something here in poor quality, can I delete it and send it again in higher quality?
and if it is deleted by an ADM, can I still repost it with better quality or will it count as spam?

Users can request access to the replacements beta - for more info, see e621:replacements_help