You can buy some cleaner such as Out Advanced Pet Urine Destroyer which will help neutralize the urine odor and remove it from the carpet.
You can also use some standard pet urine odor neutralizer in a carpet cleaner to clean the carpet with which will get rid of the cat urine smell.
Another way to get cat urine out of carpet is to sprinkle some baking soda on the affected part of the carpet where the cat urine is and let it sit for awhile.
While the baking soda is sitting in the carpet take some hydrogen peroxide and dish detergent and mix one quarter cup peroxide with a teaspoon of dish detergent and then either spray it onto the carpet or pour it onto the carpet where the cat urine is.
Work it into the carpet with a brush and then let it dry and then once it dries you can vacuum the residue out of the carpet.