Japt -hP, 16 bytes

Takes input as a 2D character array.

ÇiUËouÃíUî"_-"1c

Try it

Japt -hP, 16 bytes

Takes input as a 2D character array.

ÇiUËouÃíUî"_-"1c

Try it

ÇiUËouÃíUî"_-"1c :Implicit input of 2D character array U
Ç :Pop last element, pass it through the following function & push back
 i : Prepend
 UË : Map the remaining elements of U
 o : Modify last
 u : Uppercase
 Ã : End map
 í : Interleave
 Uî"_-" : Mould "_-" to length of U
 1 : End prepend
 c : Flatten
 :Implicitly join and output last element