Ich ändere die Signatur in der E-Mail.

Because ändern is the infinitive, meaning to change. After ich, you need the conjugated form:

• ich ändere = I change / I am changing
• du änderst
• er/sie/es ändert

So in a normal present-tense sentence, German uses the finite verb form, not the infinitive.

Signatur is a feminine noun in German, so its basic article is die:

• die Signatur = the signature

That is why the sentence says die Signatur and not der Signatur or das Signatur.

Because in can take either the accusative or the dative, depending on the meaning.

• accusative: movement into something
• dative: location inside something

Here, the meaning is location: the signature is being changed within the email, not moved into the email. So German uses the dative:

• in der E-Mail = in the email

Since E-Mail is feminine, the dative form of die is der.

Yes. In standard German, E-Mail is feminine:

• die E-Mail
• in der E-Mail
• eine E-Mail

So der here does not mean the noun is masculine. It is simply the dative singular form of the feminine article die.

Grammatically, it means in the email or within the email.

Depending on context, a learner might wonder whether it means:

• inside the text of an email message
• in the email program / email settings
• in an email-related area where the signature is edited

The phrase itself is broad enough that context decides the exact interpretation. German often leaves that kind of detail to the situation.

Because in German, all nouns are capitalized.

So:

• die Signatur
• die E-Mail

This is one of the big visual differences between English and German.
Also, Ich is capitalized here only because it is the first word of the sentence, not because pronouns are normally capitalized.

The ä is pronounced differently from a plain a. For many English speakers, it is somewhere around the vowel in bed or air, depending on accent and region.

ändere is roughly:

• EHN-deh-reh

The r in the middle is part of the stem änder-. Try to keep all three syllables:

• än-de-re

Do not pronounce it like English ander with a flat a. The umlaut matters.

German main clauses usually follow the verb-second rule. In a basic statement, that often looks like:

• Ich (subject) + ändere (verb) + the rest

So:

• Ich ändere die Signatur in der E-Mail.

If something else came first, the verb would still stay in second position:

• In der E-Mail ändere ich die Signatur.

That is a very important German word-order rule.

Yes, often you could, depending on the intended meaning.

• die Signatur = the signature
• meine Signatur = my signature

Using meine makes it more explicit that it is your own signature.
Using die Signatur can sound more neutral, especially if the context already makes it clear whose signature is meant.

Yes. German often uses compound nouns, so E-Mail-Signatur is very natural and sometimes clearer.

Compare:

• die Signatur in der E-Mail = the signature in the email
• die E-Mail-Signatur = the email signature

The compound version can sound more specific, especially if you mean the standard signature block used in emails.

ändern means to change / modify / alter something.
That fits well when you are editing the content of the signature.

wechseln usually means to switch / replace / exchange.
So the nuance is different:

• Ich ändere die Signatur. = I am modifying the signature.
• Ich wechsle die Signatur. = I am switching to a different signature.

Both can be possible in the right context, but ändern is the more direct match for edit/change.

Because German usually uses the simple present for both ideas:

• Ich ändere = I change / I am changing

German does not normally need a separate form like English am changing. Context tells you whether it means a general action or something happening right now.