How would i do that? and is there anyway the stem can get stronger without replanting?
You simply carefully remove the seedling from the soil, dig a deeper hole so when you replant it the soil level will be roughly one quarter inch below the first leaves. New roots will push out of the buried portion of the stem, so the root system will grow/expand faster.
One very simple way is if you have a narrow garden trowel you stick it in the soil a few inches away from the seedling, work it back and forth to create a space/gap, do that all the way around the seedling, then tip the pot, stick the trowel in on the low/bottom side of the seedling and slide it out, with the soil still around it.
While I do not like doing it this way, I do know people who, if their soil is very loose, will just loosen the soil close to/around the seedling, to a depth where the roots could have reached, and literally pull/slide the seedling out of the loosened soil and have a bare root seedling to then plant deeper.
Either way, once replanted deeper, keep your lighting as close to the top of your seedlings as possible without causing heat issues and add a small fan blowing on low, not directly on the seedlings but to create enough air movement so they will move/sway some, and that till cause new above soil stem growth to be thicker/stronger. It will work the very same way that wind causes trees and bushes and plants to sway resulting in thicker stems/trunks/branches.
