There is no acceptable way to prevent this. Use a different font. (It is possible that there is an extended version, with higher fee, of the font you bought.) The font should be selected so that it contains all characters, at least all letters, that you may need in the text.
It is possible to use different fonts for different letters in a word, using various techniques (@font-face
with range settings being the most elegant, but with limited browser support). However, it means a typographic disaster. Especially if the text contains e.g. both “ü” and “u”, there is usually a striking mismatch.
Editing the font itself is technically possible using a font editor, but normally illegal unless permitted in the font license or in exemptions to copyright in applicable legislation.