replace > 0 with an M

I am using windows XP, Access 2003, but my database was created in access 2000.

I have a table with data already in it, one column has dolor amounts in it, and I need to find all cells with amounts greater that 0 and replace them with an M, all other cell with 0 in them need to be blank, so the only thing I have left in this column is M's where the value was greater than $0.00.

I also have another column with values like 24B, 24C, 24G, 24A, and so on, what I need to do is eliminate the 24 and just leave the letter in each cell, is there anyway to do this without going through each cell and erasing the number.

Thanks

Sam