Fill the Blanks

There is a string $S$ of size $N$ consisting of numbers between $0$ and $9$, and some blanks represented by $?$. Let $M$ be the number of blanks. Then, the following process repeats $M$ times : - A uniformly random number $X$ between $0$ and $9$ is generated. - You choose $i$ such that $S_i$ is blank and change $S_i$ to $X$. You want to maximize the final value of the string $S$ at the end o