If you are currently under Software Maintenance Service (SMS), or if your SMS expired after March 11th 2021, then you are entitled to download and install R2021a. (When you purchase a permanent license, one year of SMS is included, including when a Student license is purchased.)
If your SMS expired before March 11th 2021, then you can contact Mathworks Sales to back-pay SMS to the current time. I do not know what the current policy is on how many years you can back-pay. Historically it used to be 4 years -- but that was for Standard (full-price) licenses, and I suspect the policy is different for Student licenses.
If you are beyond the back-pay limit, then you would need to purchase a new license.
Note: if history is anything to go by, somewhere between Monday of next week, and the equinox, Mathworks is probably going to release a new version, R2022a. There were some technical difficulties so perhaps it will be delayed.
