Acer Aspire 5810T

These are 2 totally different questions, so you get 2 totally different answers.

1. Boot from bootable flash drive
Acer's generic user guide says:
1. F2 to enter the BIOS, then in the Main page enable the F12 boot menu
2. then in the F12 boot menu select the USB flash drive
See your copy (hardcopy or pdf) for more details.
I assume you made your bootable flashdriver using the Media Creation Tool

2. https://support.microsoft.com/help/4026971/microsoft-account-how-to-reset-your-password

Since you wrote it's from Microsoft I can't answer what you have there.

This issue is not uncommon as folk use the email login system and that can break. Since it's not a good login system you are reminded to keep a spare local login that is also an administrator. Try other accounts to see you you can get in.

You can do these:
Insert USB memory stick to Acer laptop and start or restart Acer laptop. When Acer logo appears on screen, press F12 to enter Boot Device Options, and then choose USB memory stick to boot from.

And you can find options on how to boot computer from USB drive here.

As for your Microsoft account password, firstly, you can go to the official website to ask for the password reset: https://account.live.com/password/reset . Or they are many Windows password reset software that can help you to remove Windows 10 login password, no matter it is Microsoft account or the local account . You can refer to this article: https://www.lifewire.com/commercial-windows-password-recovery-tools-2626182.