I cannot seem to be able to change blocks when playing Minecraft on my laptop. I have no idea how to either. It has changed before to a different block but i have no idea how it did.
You can use either the mouse scroll wheel (or scrolling sections/gestures on a trackpad) or the number keys 0-9 on your keyboard to switch between currently selected items in your hotbar. These are the main methods of doing so.
The "Pick Block" key is by default bound to your mouse Button 3, or the scroll wheel. Since many laptops do not support clicking with the scrolling function, you can bind the control to another key on your keyboard instead.
What Pick Block actually does allows you to quickly switch to another block type if you have it in your quickbar. For example, let's say you have some stone in your hotbar. If you use Pick Block on a stone block in the world, it will quickly swap to have that item selected.
Alternatively, using Pick Block in creative mode will automatically bring that block into your hotbar, if it's not already there.
You do not have to use the mouse button 3 for changing blocks; the numbers 1 to on your keyboard will choose the respective blocks from your selected quick bar (the row of blocks on the bottom of your screen).
When you changed it to another block, you probably happened to have pressed one of the numbers on your keyboard. You should try ordering blocks consistently so that you have an idea of which number is which (for common things such as a pickaxe or stone), and from there you can get used to using numbers and quickly switching.
You can see the controls here for details.
To be able to play Minecraft without a compatible mouse (one without a scroll wheel) meaning accessing your quick-bar problematic.
The following controls are able to do the same job.
Mouse Movement Turning; aiming; third person camera movement.
Mouse Wheel Scroll Scrolls through the quick-bar.
1-9 Selects the appropriate quick-bar item.