To find and replace text in comments in Excel, first select the cells that contain the comments you want to search. Then, on the Home tab, in the Editing group, click Find & Select. In the Find and Select dialog box, click Go To Special. In the Go To Special dialog box, click Comments, and then click OK. This will select all of the cells that contain comments.

Once the cells with comments are selected, you can use the Find and Replace feature to search for and replace text within the comments. To do this, on the Home tab, in the Editing group, click Replace. In the Find and Replace dialog box, in the Find what: box, enter the text you want to find. In the Replace with:, enter the text you want to use as a replacement. Then, click Options, and select the Match entire cell contents only check box.. This will ensure that only entire cells are replaced, and not just parts of cells.

Reference: Find and replace text in comments - Office Support