I would recommend to recode the file. Before you convert the file you should make a backup!
There are several programs to do so, e.g.,
iconv. Some editors can also be used, e.g., Texmaker:
Start Texmaker and select
Convert to unicode
Select the file to be converted either by typing in the filename or better by clicking to the select icon in the top right corner and selecting it in the file selection dialog:
Click to the
Convert to unicode button:
Click to the save icon in the top left corner of the result window:
I would suggest to use a new name e.g.
test-utf8.tex and then change
If you really don't want to convert the file, you can try:
to input the latin1 file into a utf8 document. But IMHO things are more easy if all files have the same encoding.