How do I correct garbled transcription evidence on FamilySearch, used as a source for my father's first marriage in India in 1925?
If the Index allows you to Edit - you can make corrections yourself. If there is no Edit option - use the Feedback tab on the right-hand side of the page so submit feedback about the erroneous index. The index won't necessarily change anytime soon - so you can attach the record to your father's sources and in the Attach Reasons indicate the index needs correction.
I hope this helps.
It's important to keep a distinction in mind: the index is not the data. The fact that the index is garbled makes it somewhat difficult to use, but in fact, it did its job: you found the data (the marriage record) using it.
Some fields of some indexed collections on FamilySearch can now be corrected by users. If this collection is among them, then there will be an Edit button available. If not, then there's nothing you can do with the errors in the index.
Either way, attach the indexed record as a source, as genthusiast said, noting in the reason that the index has or had errors. I also suggest putting a full (and correct) transcription in the source citation's Notes field.
Many thanks for your swift reply - George Hamber (York, UK)