When I need to reset a forgotten or unknown password on a Microsoft Windows system, I use the Offline Windows Password & Registry Editor. You can download the software from Offline Windows Password & Registry Editor, Bootdisk / CD.
You will need to create a bootable CD or USB flash drive from the .iso image file within the zip file you download and then boot the system from the CD or USB flash drive. Once you've done so, you can remove the password for any account on the system. Note that simply copying an executable file to a CD or flash drive as you might usually do to put files on disc/disk is not sufficient. You have to create bootable media so that you can boot from that media rather than booting the system into Windows. You can find instructions for creating a bootable CD in responses to another question posted on this site, Create Bootable CD from ISO.
Some systems will boot from a bootable CD or flash drive, if one is available, before booting from an internal drive. Others will attempt to boot from an internal drive first. If that is the case with your system, you may need to change a BIOS/UEFI setting to configure the system to attempt first to boot from a CD or USB-attached device. To change the setting, if the system won't attempt to first boot from those media, you will need to hit a key or key combination when you restart the system before it boots into Windows to get to a setup menu. That key or key combination will vary depending on the BIOS manufacturer, but you should hopefully see a message appear on the screen before it boots into Windows informing you what key or key combination you would need to use to get to the system's setup menu.
There is an overview of the process for resetting an account's password at
Offline NT Password & Registry Editor, Walkthrough. You can also find other guides for using the tool online, such as Reset Windows 8.1 password with the free tool ntpasswd at the 4sysops site. The article explains how you can create a bootable disc, if you don't know how to do so. The author of the article, Michael Pietroforte, also has posted a list of other articles he has written for resetting a forgotten administrator password on a Windows system at Reset Windows 7 / 8 administrator password
I haven't yet needed to use the software on a Microsoft Windows 10 system, and the download page states "More or less tested from NT3.5 up to Windows 8.1, including the server versions like 2003, 2008 and 2012. Also 64 bit windows supported.", so I'm not certain it will work on a Windows 10 system, but I've used it for many cases where someone has forgotten a password for an administrator account on prior versions of Windows.