You probably come across numbers quite frequently in your writing. If they are part of a keyword, do not change them! Otherwise, here are some tips to help you understand when to use figures (such as 1, 2, 3) and when to spell out numbers (one, two, three).
Always use figures for numbers 10 and higher, and for fractions and decimals. Use figures for percentages but spell out the word “percent.” Also use figures when referring to ages, dates (with the month spelled out) and physical dimensions, as well as for naming decades and centuries.
Words should be used for numbers one through nine, and for any number beginning a sentence. Also use words when you are spelling out very large numbers such as million or billion. If you come across anything that is not covered here, an AP style guide is your ultimate resource.