Learn Italian: Il Suo aiuto è molto importante, grazie. - Your help is very important, thank you.

Capitalizing Suo shows that it means the formal 'your' rather than 'his' or 'her'. When using the formal Lei register, writing it with a capital letter is a polite way to make it clear you are speaking directly to the person.

If I am thanking a woman, do I need to change it to La Sua?

No, it stays Il Suo. In Italian, possessive words must match the gender of the thing being owned, not the person you are talking to. Since aiuto (help) is a masculine word, you must use Il Suo whether you are formally addressing a man or a woman.

Why is there an Il in front of Suo? Doesn't that mean 'the your help'?

Yes, it literally translates to 'the your help'! In Italian, you almost always need to put a definite article (like il or la) before a possessive word like Suo. English drops the 'the', but Italian keeps it.

Why does importante end in -e? Shouldn't it end in -o to match the masculine noun aiuto?

While many adjectives change to -o or -a depending on gender, there is a group of adjectives that naturally end in -e in the singular. importante is one of these, so it keeps the -e ending for both masculine and feminine nouns.