How to I clean my towels to get rid of a mildew smell?

Use some vinegar in the rinse cycle.

Plain old white vinegar. And skip all the chemical filled products. Use vinegar in the wash, in the rinse cup and felted wool dryer balls. NO FABRIC SOFTENER. If you live in a humid climate, you might need a dehumidifier or use damp rid type products.

White cleaning vinegar - soak towels before hand or add to rinse cycle.

Great. Thanks for all your replies. Vinegar seems to be the theme here so I will try that. By the way, I live in Scotland so definitely not a humid climate. Just wet which could make it difficult to dry towels in winter. I tend not to use my tumble dryer. Should I? Only for the towels?

Fabric, especially porous fibers such as cottons found in toweling, will quickly acquire an unpleasant smell if there is insufficient air circulating through the fibers during the drying process.

No amount of washing in vinegar or any other solution will ensure a nice odour if the towels:

1. sit in the washer wet before they are dried ,or
2. sit ,or hang without moving and are partly dried by any means before being fully dried afterwards

As we are now entering into the winter season when it is not as convenient to have windows open or to hang the towels out on a line where they will have robust warm air, yes, treat yourself and use the tumble dryer!

Scotland does not have a high humidex, as you don't have moisture and heat at the same time, but the dampness is coming from the moisture in the air.

I just checked two different locations in Scotland on The Weather Network and both were recording 87% humidity.

Use the dryer for the towels for starters, and see what difference this makes in how they smell. Then decide whether you want to use it more for drying other items such as sheets and so forth. It is your choice. It may take a couple of rounds of the washing/drying cycle to entirely remove the vinegar smell if it has been used aggressively.