These days, I just compost it, although sometimes, I use it to fill in the doggie-created craters in my yard.
In the old days, I just did cold composting, so I had to be careful about tossing in weeds, or anything diseased. If the used potting soil was from a plant that had died of a disease, or if the soil was infested with vermin such as fungus gnats, I would first sterilize the soil in my kitchen oven. Then
I would toss it on the compost pile to be renewed with good soil denizens.