Step 2 for HP Pavilion Chromebook in this guide. Above guide contains information and links on upgrading memory and storage as well!

Step 2 for Lenovo Thinkpad X131e Chromebook... Find the write protect switch inside the case on page 14 of this pdf file. Requires some disassembly of course. Good luck

Step 2 for many other Chromebooks involves removing a write protect screw somewhere on the motherboard. Google is your friend here.

Example write protect screw on HP Chromebook 11 G3 (2013) HP Chromebook 11 G3 write protect screw specifics

Since you have it apart this is a good time to upgrade to more memory (if your chromebook model supports that) and or a larger SSD if you wish or need more space for dual boot for example. Reassemble carefully in reverse order.

Boot into chromeos for next step.