That would be an election, and currently that isn’t doable according to the Ukrainians, while they are under attack, large portions of the nation occupied and being destroyed, no electricity, resources, manpower etc..
The main point though is that he is still legally in office, the law against elections during martial law maintains a legal government during war.
That's according to the Ukraine Parliament that created and passed the original martial law as part of the Constitution, not the Ukrainian people. The martial law part of the Constitution has been reformed several times over the years.
Declaring martial law gave Zelensky power to do things he wouldn't have normally been able to do, like force all national TV stations to merge into one platform, limit the right to freedom of expression, peaceful assembly and freedom of movement, close all churches affiliated with Russia, thus taking away individual rights to practice their religion at a religious institution of their choice, enforce a curfew that has since been lifted, ban all men of the ages 18-60 from being able to leave the country, and suspend the activities of 11 political parties. On May 22nd, the day after Zelensky's term of office expired, Ukraine's Parliament extended martial law for another 90 days. I bet the people of Ukraine are just loving it.