#SmileBASIC, 27 25 bytes

DEF D S
S[0]=@1[S>"2"]END

Creates a function named D that is called like:

STRING$="abcde"
D STRING$
'STRING$ is now "1bcde"

The first character is replaced with either @ or 1

SmileBASIC, 27 25 bytes

DEF D S
S[0]=@1[S>"2"]END

Creates a function named D that is called like:

STRING$="abcde"
D STRING$
'STRING$ is now "1bcde"

The first character is replaced with either @ or 1

#SmileBASIC, 27 bytes

DEF S S S[0]=@L[S[0]<@L]END

Creates a function named S that is called like:

S$="abcde"
S S$
'S$ is now "@bcde"

The first character is replaced with either @ or L

#SmileBASIC, 27 25 bytes

DEF D S
S[0]=@1[S>"2"]END

Creates a function named D that is called like:

STRING$="abcde"
D STRING$
'STRING$ is now "1bcde"

The first character is replaced with either @ or 1