Pour a little bleach in them. Leave to sit. Scrub gently. Rinse. Repeat if necessary (unlikely). Wash thoroughly.

I write this because for a long time I used bicarbonate of soda, or salt, or some productised kitchen cleanser, or even just washing up liquid and a lot of scrubbing, which is the sort of advice you find if you search the internet for "how to clean tea stains off mugs". None of these options is any good and I think people only suggest them because it feels weird bleaching something that you will be drinking from, even though all the bleach has gone after washing.