You passed 12th in 2024 as a PCB student and now plan to appear again in 2026 with PCM through NIOS.
For JEE Main 2026, you will be eligible because students can appear in the year they pass 12 and up to two more years.
Since you are giving the 12 again in 2026, you can apply for JEE Main 2026.
For JEE Advanced, eligibility is based on the year you first passed Class 12.
Usually, you can attempt JEE Advanced only in the two years after your first 12th pass (2024 and 2025).
Giving 12th again in 2026 normally does not reset JEE Advanced eligibility.

Hope this helps you