If word has letter than change in Excel

In Excel I have this formula that I need it to identify in a certain cell if the last character is the letter "o", and if so, change it the letter "a".

Example: if the word is "Vermelho", it changes to "Vermelha".

And it works as it is.

=IF(RIGHT(R173;1)="o";REPLACE(R173;LEN(R173);1;"a");IFERROR(LEFT(R173;FIND((" ");R173)-1);R173))

I also need the formula to extract only on word, 'cause if it only contains one word, it works fine, but if it contains more than one word, it doesn't work.

Example: if the word is "Vermelho and Amarelo", it changes to "Vermelho and Amarela". I tried the following If Error, =IFERROR(LEFT(H4;FIND((" ");H4)-1);H4) but I can't them to work together.