A cell stores a value and displays it using a number format. The number 0.08 can appear as 0.08, 8%, or $0.08. Changing the format doesn't multiply it or turn it into text. The formula bar still shows the value used in calculations.
Select a cell or range, then use the currency, percent, or decimal buttons on the toolbar. The 123 menu has Number, Currency, Percent, Date, and custom formats. A date is a day number underneath, so formatting a date as Number shows its serial number.
Check the starting value before applying Percent. A stored 8 becomes 800%; a stored 0.08 becomes 8%. Decimal buttons round the display only. An amount shown as $12.88 can still use 12.875 in a multiplication. Keep quantities as numbers and use currency for money.