Find out where the leak is coming from first. Slow leaks can be from around the rim-especially if there is some rust on the rim, from the tire valve core or around the valve itself, from a small nail, or even a leak in the wheel itself. Mix a little dishwashing detergent with water and flush the tire looking for bubbles to find the leak. For a slow leak it may take a while for the bubbles to form. On hard to find ones, I’ve pulled the wheel off and flooded everything to find a small leak around a valve. The remedy is new valve, reseating the tire, etc. depending on where the problem is. If the tires are worn though replace them.