Terminology:  utf8 is a CHARACTER SET, not a COLLATION.
It is unclear what problem you are trying to solve...
- Perhaps you mistakenly stored utf8 characters in a latin1 field?
- Perhaps you want to re-declare tituloas utf8?  (ThatUPDATEwill not achieve that; it will make a mess.)
- Perhaps titulois already declared utf8, but data was stored incorrectly?
- Something else??
See Trouble with utf8 characters; what I see is not what I stored  In particular, see the SELECT HEX... tip.  Please provide the output from such, so we can figure out what got stored.  Also provide SHOW CREATE TABLE documentos2 so we can know what titulo is currently declared as.