add underscore in between letter and number

I dont know very much about excel and have searched all over Google for half the day tired of wasting time.
I have hundreds of columns that look like this

01A
01B
01C
01D
02A ect.....

I need to add an underscore in between the letter and number like this

01_A
01_B ect......

any help would be greatly appreciated,

Your post does not comply with Rule 1 of our Forum RULES. Your post title should accurately and concisely describe your problem, not your anticipated solution. Use terms appropriate to a Google search. Poor thread titles, like Please Help, Urgent, Need Help, Formula Problem, Code Problem, and Need Advice will be addressed according to the OP's experience in the forum: If you have less than 10 posts, expect (and respond to) a request to change your thread title. If you have 10 or more posts, expect your post to be locked, so you can start a new thread with an appropriate title.
To change a Title on your post, click EDIT then Go Advanced and change your title, if 2 days have passed ask a moderator to do it for you.

Thanks for fixing the title...

are the consistently 2-digits, underscore, 1 or more letters...

If yes,

try =REPLACE(A1,3,0,"_")

where A1 contains first item to change...

You can then copy|Paste Special >> Values over original if desired.

You need to enter the formula in a cell adjacent to the top-most code... then copy that formula down by double-clicking the little black square at the bottom right corner of the cell with the formula in it. It should create the new code with the underscore in each cell below.

Then you select this new column, copy it... go to the old column and go to Edit|Paste Special and select Values.

it should look like this:

01_A
01_B
01_C
01_D
02_A
etc...

See attached..

First I entered formula in L4 and copied it down to L11.

Then I selected and copied L4:L11.

Went to J4.

Went to Edit|Paste Special and selected Values.

Click Ok and presto.

Now you can delete column L entries.

If you are satisfied with the solution(s) provided, please mark your thread as Solved.

How to mark a thread Solved
Go to the first post
Click edit
Click Go Advanced
Just below the word Title you will see a dropdown with the word No prefix.
Change to Solved
Click Save