Changing a value if it’s under some number to a certain number, and keeps the value if it’s above.

Hi.

I am making a table about school grades, and I want to change every grades under 70 to 70, but keeps the same number if it is above 70.

(e.g.: if the value is 50, then the result is 70. But if the value is 89, then the value will still at 89). 

What kind of formula should I use?

Thanks in advance!