You can easily replace all-enter characters with space or comma by the Find and Replace feature in Excel. Please do as follows:
1. Select the cells you will replace the alt-enter characters, and press Ctrl + H keys to open the Find and Replace dialog box.

2. In the Find and Replace dialog box, under the Replace tab, please:
(1) In the Find What box press Ctrl + J keys to enter alt-enter character;
(2) In the Replace with box type space or comma as you need;
(3) Click the Replace All button. See screenshot:

3. Click the OK button in the popping out Microsoft Excel dialog box. And then close the Find and Replace dialog box.

Now you will see all alt-enter characters are replaced with your specified character, such as a comma. See screenshot:

Combine rows/columns into to one cell and separate with space, comma, new line, etc.

Kutools for Excel’s Combine Columns or Rows utility can help Excel users easily combine multiple columns or rows into one columns/row without losing data. In addition, Excel users can wrap these combined text strings with carriage or hard return, comma, space, semicolon, etc.

If you are familiar with VBA, you can also replace the alt-enter characters with space, comma, or any other character as you need with VBA in Excel.

1. Press Alt + F11 keys to open the Microsoft Visual Basic for Applications window.

2. Click Insert > Module, and then paste below VBA code into the new module window.

VBA: Replace alt-enter with space/comma in Excel

Sub ReplaceLineBreak()
Dim xRg As Range
On Error Resume Next
Set xRg = Application.InputBox("Select Cells:", "KuTools for Excel", Selection.Address, , , , , 8)
If xRg Is Nothing Then Exit Sub
xRg.WrapText = False
xRg.Replace Chr(10), "Your text here", xlPart, xlByColumns
End Sub

Note: Before running the VBA, please find the code xRg.Replace Chr(10), "Your text here", xlPart, xlByColumns, and replace Your text here with space, comma, or other character as you need.

3. Press F5 key or click the Run button to run this VBA.

4. In the popping out KuTools for Excel dialog box, please select the cells you will replace the alt-enter characters, and click the OK button. See screenshot:

Now you will see all alt-enter characters are replaced with you specified character, such as comma. See screenshot:

How to replace blank space with nothing/underscore/dash/comma in Excel?