Is there any way to change the "remaining" time of a giveaway, after it's created?

not once created :(

You can't. And if you would delete them now, you'll lose 1 giveaway slot per deleted giveaway so in the end you won't be able to create new giveaways (as a new user you have 3 giveaway slots). Just wait those 4 weeks ;)

I recommend just waiting out that time. I ran into the same problem with my first giveaway D:

Btw Support is really slow at answering tickets.

What others said. Also, thanks blarkio for giving and also for asking in the forum rather than just spamming support many times or deleting those GAs and asking support for more slots.

CG can, but he does have other stuff he works on, so might not happen.

He did it once for me though. :D

I saw that you already ok with waiting, but while you're waiting, you could create a different support ticket, asking if a mod would manually increase your giveaway slots.

If they see your ticket within 3 weeks and agree, you win.
If they see your ticket within 3 weeks and don't agree, you lose nothing.
If they don't see your ticket within 3 weeks, you can close it. Again, you lose nothing.

by the way, after your 3 giveaways successfully end (the winners need to mark them as received), you will have 4 slots (you gain 1 slot for 3 successful giveaways).