I realize there is the old way of doing things, that was back when people like me were desperately working with mud trying to invent the stone so we could celebrate entering the stone age and sheets of paper glued into piles was how information was kept together, and there is the new way of doing things, where information is communicated directly to your brain using holograms and smell alone. If you have to go looking for a video then I wish you luck, but you didn't seem to indicate that you had had any luck with the new ways.
New purchase, new install, continual crashing so contact customer support directly and see if they can help you get that solved.
When you have really understood substitutions and can effectively use those within Mathematica then you might find that it can be easier, faster and less error prone than cut and paste with your mouse. You can name very large things and refer to them just by their name.
Perhaps depending on which version you have you may see Evaluation on the menu bar at the top of your window. You can click on that and then see the drop down menu listing Quit Kernel. Clicking on that and then on Local and then on Quit will tell Mathematica to discard all previous stored results. The calculations displayed in the screen will still be there, but any stored values from your previous calculations will be lost. And clicking on Evaluation and then Evaluate Notebook will step by step do every single step in your notebook. To tell Mathematica to "forget" the stored value of eq1 try this
eq1=.
What is written in the notebook can be very different from what Mathematica has been previously told to store in memory. This often causes confusion for new users.
Everything I have written here and thousands more things will be described in some online video somewhere. Maybe try Google with
Mathematica video introduction
and see if you can find something. That seems like what I would have tried.
Lastly I will say, there is the "Mathematica way of thinking" and until you have come to understand some of that you may find Mathematica to be confusing and aggravating. After you have learned some of that then it may be confusing and aggravating, but for much higher reasons. And repeat. And repeat.
I hope it works out for you. I'd start by contacting customer support until you get the crash problem fixed.