My 91 Honda Civic transmission slips when going from 1st to 2nd.Whats the cause? on 1991 Honda Civic

Problem just started happening. Haven't had prior problems with transmission. Mileage is at 224,000.

Asked by for the 1991 Honda Civic
What trans. Auto or manual?
When is the last time you had the fluid changed? At any time did you notice the "D" light flashing? There may be trouble codes stored in the trans control module , might want to have it scanned at a shop with a scanner that can communicate with it.(generic code readers won't do it) Use only Honda ATF. Drain & refill only -NO FLUSH! If fluid is really old & burned , Do 1 drain & fill(about 2&1/2 quarts) , drive it for maybe 20 miles & drain & fill again.
Oh okay. Its been two years since I've had my transmission drained and refill. The D light has not flashed.How soon do I need to have it drained and refilled?
Every 30K has been a good interval ,from my experience.