When you enter data into Excel, it formats it according to what it interprets as the data type you intended and normally does a great job of making the right guess. For instance, if you type 3:27, it assumes you mean for this to be a time entry and formats the cell accordingly. Without specifying, however, it doesn't know if you mean AM or PM and will assume AM. There are three ways to change the time to PM. Two involve manually changing the input, and the third, which works best for a large number of cells, involves creating a formula.
Place the cursor on the cell where you want to change the time from AM to PM. Enter the time in 24 hour notation. For example, instead of 3:27, enter 15:27. Press "Enter" to make the change.
Place the cursor on the cell where you want to change the time from AM to PM. Re-enter the time but leave a space after it and type in PM. Press "Enter" to make the change.
Place the cursor in an adjacent row or column to the cell you want to change from AM to PM. Type "=" and then use the arrow key to move the cursor onto the cell with the AM time. Type "+ .5" and press "Enter" to finish the formula. In Excel math on dates and times is done in increments of days, so you have added half a day, or 12 hours, to the original cell. Use copy and paste to copy this formula if you have multiple cells in which you want to change the time from AM to PM.