But if the words are constant and are unlikely to change you might like to consider using a Regex Replace activity.
Use the following in an Assign Activity:
yourString = System.Text.RegularExpressions.Regex.Replace(INSERTVARIABLE, “(?=\sUse the spare part solution)”, “:”) Regex101.com solution preview
Now it depends on the data where you have to write it.
If some data is already present and then you have to write then you can use write cell Activity or else Build Datatable Activity, Add Data Row Activity and then Append Range Activity.
If you have to write the data newly to the sheet then you can also use Build Datatable Activity & Add Data Row Activity and then Write Range Activity.