Please follow this link: https://askubuntu.com/questions/1031993/how-to-install-ubuntu-18-04-alongside-windows-10

Fixing Dualboot after Windows update

After updating Windows, you may notice that Grub is missing or launchs into Grub terminal. This fix mainly applies to the latter, but should work for the former too but will require additional research.

1. If you launch into grub terminal, type exit to exit it and boot into Windows.

2. Go into your BIOS and disable secure boot. This can be done by holding shift then restart. Afterwards, go to Troubleshoot > Advanced Options > UEFI Firmware Settings and then click Restart. See: https://docs.microsoft.com/en-us/windows-hardware/manufacture/desktop/disabling-secure-boot#:~:text=Go%20to%20Troubleshoot%20%3E%20Advanced%20Options,Save%20changes%20and%20exit. for more details. Once in BIOS, make sure secure boot is disabled. Note: you may also have to make sure fast boot is disabled too. If you can't change/select the secure boot option, then you'll have to look into what to do for your particular laptop. For Acer laptops, you can set a supervisor password (e.g. pick 1234), then disable secure boot then clear the supervisor password by inputting your current password and leaving the new password field blank.

3. Make sure you have a bootable USB/SD card with Ubuntu installed. If you don't, get a USB that's at least 2GB in size. Afterwards, download the Ubuntu version you need (See: https://ubuntu.com/download/desktop). You should have a .iso file or an image file of the Ubuntu OS. Now, use software to write the iso file into your USB. Make sure you select the correct drive otherwise you may overwrite something important. In particular, avoid writing to your computer's HDD or SSD. For software, I've tried balenaEtcher and Win32DiskImager. See this page for how to use the tools: https://www.raspberrypi.org/documentation/installation/installing-images/windows.md.

4. Now restart your computer and boot into your USB. What I personally ended up doing was restarting my computer which opened to the Grub terminal. Afterwards, I typed exit on the terminal and had the option to boot into 3 different OSes: Windows Boot Manager, ubuntu and USB Mass Storage Device. I selected USB Mass Storage Device. The exact steps you may use will vary, particularly if you can't access grub at all.

5. Now you should see the grub menu open. Select Try Ubuntu without installing. This should launch you into a vanilla Ubuntu OS. Now, connect to the Internet. Once you have Internet, you should install boot-repair. This can be done by first opening a Terminal window. Once you have a Terminal, type this command sudo apt-add-repository ppa:yannubuntu/boot-repair, followed by sudo apt-get update. Now, type sudo apt-get install -y boot-repair. Congrats, you have boot-repair now. See: https://techwiser.com/fix-grub-after-windows-10-upgrade/ for more details.

6. Now that you have boot-repair, type boot-repair in the Terminal window to open boot-repair. It might take a while for it to scan everything. Once that's done, select Recommended repair (repairs most frequent problems) in the GUI that opens. Afterwards, boot-repair should attempt to fix your boot. Once it's done, make sure the process ran successfully, and then reboot your computer. Personally, once this was done, I had no troubles (e.g. grub was working fine again). If boot-repair gives any errors, contact a team member for support. If not, congrats, you should be able to boot into Windows and Ubuntu once again!