Just highlight the cells you want changed (or entire worksheet by pressing
the grey box between Row 1 and column A). the go to edit Menu - Replace.
Enter in From the period and the to box a colon. Press replace all.

"mailrail" wrote:

One of my data sheets at work brings numbers into Excel as decimals. For
instance, 5 hours and 39 minutes is shown as 5.39. How can I get Excel to
convert this to 5:39 (semicolin instead of decimal)?