Activate Windows file lost during upgrade to Win 10 - How do I obtain activation code?

Device: Dell Optiplex 755 Core 2 Duo vPro

Operating System
Windows 10 Enterprise 64-bit
CPU
Intel Core 2 Duo E8400 @ 3.00GHz 38 °C
Wolfdale 45nm Technology
RAM
4.00GB Dual-Channel DDR2 @ 332MHz (5-5-5-15)
Motherboard
Dell Inc. 0GM819 (CPU)
Graphics
HD2250 (1680x1050@59Hz)
1024MB NVIDIA GeForce GT 220 (NVIDIA) 48 °C
Storage
232GB Western Digital WDC WD2500AAJS-75M0A0 (SATA) 37 °C
149GB Seagate ST3160815AS (SATA) 34 °C
7GB USB Flash Memory USB Device (USB)
Optical Drives
TSSTcorp DVD+-RW TS-H653F
PLDS DVD-ROM DH-16D3S
Audio
High Definition Audio Device

I need to modify BIOS ( activate VT-x ) but I can't access. Also constant clanging nag and other problems.

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