(Sorry, I was going to walk you through the way to do it in Microsofts Photo Editor, but turns out we don't even have it installed on this computer. Therefore generic tips

try and find out if you have some kind of photo editing program on your computer. Start it, and try opening one of your photos from inside. Then look over the program menus and see if you have an option called "resize".

If you have an option of resizing in cms, inches or pixels, choose pixels. Then type in the size you want, typically you only type in one and the other one changes automatically, to keep the height/width ratio the same.

Then save it under another name!
(If you see something called compression ratio turn it up to max. Jpg-format can be heavily compressed but the quality decreases.)