I’ve noticed that I’ve recently gained weight on my underarms. I do power-walking regularly to keep fit but would like to have more strength in my arms. What do you recommend?

It's great that you are keeping fit through regular, fast-paced walking but if you are looking to tone up your arms a little, you may want to incorporate some strength exercises as well. 

You could purchase some small weights, for example, and lift for 5-10 minutes in the morning or evening. This is a good option if you are quite busy during the day and find it hard to add in additional exercise. Alternatively, I am sure your local gym will have plenty of weights and other equipment to help tone up the arms if this is something you want to try. 

Other exercises that are good for toning up the muscles include swimming and Pilates.

For more information check out my blog 'A beginner's guide to strength training'.
I hope there are a few options here that you can try - good luck and have fun!