How to mark mutation sites?

Question

[mbk0asis]

Hello, everyone!

I have DNA sequences and mutation (SNP) position data like below

        sequence                 SNP_position
   atgttggagatctgcctga             3,9,15

I want to mark (mask?) SNP's with an arbitrary string such as 'S' to look like below

atSttggaSatctgcStga

I tried to find tools or codes, but haven't found one.

Has anyone done this before? If you have, please help me.

I don't know where to start.

Thank you!