How can I replace a specific character in one string, with a specific character from another string? | s1=${s1:0:2}${s2:3}${s1:3}

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ubu1404@ubuntu1404lb01:~$ s1="abcd"
ubu1404@ubuntu1404lb01:~$ s2="xwyz"
ubu1404@ubuntu1404lb01:~$ s1=${s1:0:2}${s2:3}${s1:3}

ubu1404@ubuntu1404lb01:~$ echo $s1
abzd

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