I want to learn if it's possible to manually update mods so I can breathe life into old ones like Asgard Shield which will be amazing for 1.12
Short answer: yes.
Long answer: you need to check the permissions the original mod author has given about using the mod. Then, using Java, re-write the mod into the MC/Forge version you want.
"We live on a placid island of ignorance in the midst of black seas of infinity, and it was not meant that we should voyage far."