Need to remove the diff, carefully grind out the old pan and weld in a new one.
Be fastidious about cleaning all the swarf out from inside the housing before re-assembly.
Welding the new pan in is tricky - you're seam welding thin sheet steel to a thick cast housing and you musn't let the new pan get warped.
Some people tack the new pan in 4 places and then fill the rest of the seam with JB Weld - that seems to work OK.