This should do that for all cells down to the last populated cell on your worksheet.
Dim lcell As Range
Dim cell As Range
Application.ScreenUpdating = False
' Find last cell on worksheet
Set lcell = Range("A1").SpecialCells(xlLastCell)
' Loop through all cells in workbook
For Each cell In Range("A1:" & lcell.Address)
' Change date format
If cell.NumberFormat = "yyyy-mm-dd" Then cell.NumberFormat = "dd/mm/yyyy hh:mm"
Application.ScreenUpdating = True
the newer Excel versions have the option under find (CTRL-F) or find & replace (CTRL-H) to find & replace formats
using the macro recorder on that gives some code such as below. as you can see, no loops needed