Bir Fonksiyon oluşturursunuz

Function degisHrf (intSira as integer, txtMtn as string,txtBul as string, txtDgs as string )

Y=0

For x=1 to intSira

y=instr(y+1,txtMtn,txtBul)

Next x

degisHrf=left(txtMtn,y-1) & txtDgs & mid(txtMtn, y+len(txtBul))

End function

şeklinde  dener misiniz?
Tablette yazdim, Deneyemedim hatalar olabilir

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intSira as integer, txtMtn as string,txtBul as string, txtDgs as string
intSira : 2,
txtMtn : "eskisehir"
txtBul :"S",
txtDgs :"Ş"