Hi Louise, thanks for getting in touch :) You sure can update those images yourself. If you go to Site Settings, then click Manage Assets next to the Normal theme and double click the 'slides' folder you'll see the slides there. Make sure your images are resized and cropped to 980 × 250 pixels (width x height) first, then upload them there. You can delete slides you don't want in there also.
We do offer slide creation as a service for $50 +GST. You just need to send us through enough images to make about 10 slides and we'll do the rest. Let me know if you'd rather do that instead.
I thought I'd also quickly mention our redesign service: https://www.spikeatschool.co.nz/redesign/
Have a look at that page if you're interested in giving your Spike website a freshen up. There's plenty of examples of our recent designs on that page also.
Have a great day!
Founder | Spike@School
Level 1, 6 Garden Place, Hamilton 3204
PO Box 303, Waikato Mail Centre, Hamilton 3240, New Zealand