Transmission slipping can be caused by low transmission fluid, clogged or dirty transmission fluid filter, bad transmission fluid or even a bad transmission itself.
You should first check your transmission fluid level to be sure you have enough transmission fluid.
Also smell the transmission fluid to see if the transmission fluid is burnt. If the transmission fluid smells burnt you need to have your transmission checked out because the transmission fluid should not have a burnt smell to it.
You can also add some Lucas Transmission fix to the transmission fluid as that usually helps temporarily fix a slipping transmission.
But you do need to take the vehicle into a transmission shop as soon as possible so they can check the transmission out because you might need a transmission rebuild done before you're stranded on the side of the road due to a complete transmission failure.