Pass All Table Values Into Function
Verify your intended change using SELECT

SELECT
    fullname = dbo.fnConvert_TitleCase(fullname),
    saddy = dbo.fnConvert_TitleCase(saddy)
from
 @Garbage


and then, if you're satisfied, run the update below

update @Garbage set 
    fullname = dbo.fnConvert_TitleCase(fullname),
    saddy = dbo.fnConvert_TitleCase(saddy)


That being said, scalar functions don't really perform very well - a better solution would be to create a table value function and then you could use CROSS APPLY