Mira te dejo un ejemplo, espero que te sirva:
Código vb:
Ver originalDim eqstr as integer
Dim pos as integer
Dim mystring, str as string
Dim seqstr as string
mystring="HOLA MUNDO"
str="MU"
eqstr=512
sqestr=CStr(eqstr)
mystring=replace(mystring,str,sqestr,,,vbTextCompare)
msgbox mystring