ok. I posted about my Mint 64 bit Cinnamon Sonya 18.2 getting glitchy. The best response I got was to reinstall. This machine came preloaded so I made a usb stick with Mint 20 Ulyana on it. The only instructions I can find for installing it are for installing on WIN or MAC. How to install this and overwrite my 18.2?
Thanks!

First thing you need to do is run the liveCD for a while and make sure everything works as it should - especially video and network. Re-installing will erase everything - all your data and installed programs, as well as the shipped system. Might seem obvious, but worth remembering.
After you're happy, simply follow the Mint install guide you were referred to in the other thread, and choose "Erase disk and install Mint". Just follow the guide.
If you want to save anything, do that before you start.

I have moved all essential files to Dropbox. The download was checked as part of the download process. What is the command to start Mint 20 from the USB? Yes, I really am new at this. Took a class but it has been a while.

https://linuxmint.com/documentation.php
And yes, your questions are answered there, in many languages.

I have been reading the documentation. The problem has been that the BIOS screen flashes so quickly and the commands are in such small font I have been missing them. After several tries (F10 not always responding) I have booted from the USB. It seems to be working well. I am ready to install and have another question. I have the option to install 20 alongside 18.2 in order to retain some files, like pictures and software I have installed. Will I be able to access them from 20 or will I have to start in 18.2 when I want to access them? In other words, will 20 find the Pictures folder that already exists or will it create its own?
Thank you for your patience.

Weavery,

Do you have a separate Home partition containing your documents, photos and other data?

If so, you can select Something Else as opposed to the "Alongside" option and follow the instructions here:
https://www.linuxquestions.org/quest...-distro-37074/

It is always a good idea to backup all personal data to an external drive just in case there are problems.

Thank you, I had so little I just used DropBox and a thumb drive. I am following along in the documentation as I do this. I am sure there will be more questions. Thank you all for your prompt replies.

Ok, I have a problem. I have run the installation twice now, each time asking to erase the disk and install 20. Instead it installed 20 beside 18.2. When I restart I am in 18.2 a d am not given a choice which system to run. ???

I wonder if you have a locked system so the update to the default boot gets rejected. If you have instructions to get into the firmware (aka BIOS screens) do that and have a look around - you might be able to reset the option. From a browser, go here, download and run the script and post the RESULTS.txt file it generates. It should allow us to see what if anything happened.

I went into BIOS and changed the boot order and now it boots to 20. I am sure this is hardly an elegant install but it works. And Ulyana is a lovely upgrade, works smoother than 18.2 and is visually more pleasing. This all started because I am taking a class in Mint and features in 18.2 had quit working. I can finish the class now. Thank you for your help.

Nothing inelegant at all - quite common in fact.
If you want later to recover the disk space 18.2 is occupying open another thread.