Hi,

For appearing in Neet ug, the eligibility criteria requires a minimum of 50% aggregate in 10+2.

Since, you've already have your improvement examination, you should use the Roll number mentioned in the admit card for your improvement examination while filling your neet form.

While filling the code in neet ug form, you need to opt for code 1 which is fir 13th appearing candidates.

Hope it helps.