200 A fuse etc

[Capt Ric]

Foolishly allowed drains to block on my normally garaged car which has recently been outside under trees.
Lower fuse corroded and possibly one above too.
Can anyone confirm it is 200 A part N10525605 ? what is the upper one? Do I need to change fuse holder as well or just fuse?
What precautions? Presumably disconnect battery first - anything else?

Was looking for sourse of coolant leak, which doesn't appear to be from rad or lower hoses.
Can anyone tell me what the small pipe from bottom of coolant resevoir is? Vent / overflow?

Thanks in advance
Ric