Si quieres cambiar la cita, dímelo antes.

Because quieres is the tú form of querer in the present tense: tú quieres = you want.

So si quieres... means if you want...

This sentence is speaking to one person informally. If you were speaking more formally in Spain, you would say:

• Si quiere cambiar la cita, dígamelo antes. = if you want to change the appointment, tell me beforehand.

Because after querer followed by another action done by the same person, Spanish normally uses the infinitive.

So:

• quieres cambiar = you want to change

The structure is:

• querer + infinitive

Examples:

• Quiero comer = I want to eat
• ¿Quieres venir? = Do you want to come?

You would use que + subjunctive when the second verb has a different subject:

• Quiero que vengas = I want you to come

But in si quieres cambiar la cita, the person who wants and the person who changes are the same person, so cambiar is correct.

La cita can mean different things depending on context:

• appointment
• meeting
• sometimes date in a romantic sense

In this sentence, it most naturally means the appointment or the scheduled meeting.

So cambiar la cita usually means change/reschedule the appointment.

In Spain, context will usually make it clear whether it is a medical appointment, business meeting, or something else.

Because this is an affirmative command, and in Spanish, object pronouns are attached to the end of affirmative commands.

Breakdown:

• di = tell (informal command of decir)
• me = to me
• lo = it

So:

• di + me + lo → dímelo
• literally: tell it to me
• natural English: tell me

Spanish often includes pronouns more explicitly than English does.

The accent mark is there to keep the correct stress after the pronouns are attached.

Without pronouns:

• di is one syllable

With pronouns:

• dímelo has more syllables: dí-me-lo

The written accent shows that the stress stays on the first syllable.

This is very common with commands plus attached pronouns:

• hazlo
• dime
• dímelo
• cómpralo

Because di is the informal affirmative command of decir.

So if you are telling one person informally to do something, you use:

• di = tell

Examples:

• Di la verdad. = Tell the truth.
• Dime. = Tell me. / Go ahead, I’m listening.
• Dímelo. = Tell me it.

It is irregular, so it does not follow the normal pattern of regular tú commands.

Lo means it.

Here it refers to the whole idea of changing the appointment.

So dímelo literally means:

• tell it to me

In natural English, we would usually just say:

• let me know
• tell me

Even if English leaves it out, Spanish often keeps the object pronoun.

Yes, but the meaning and tone are slightly different.

• Dímelo antes. = Tell me beforehand. / Let me know beforehand.
  This is a command or direct instruction.

• Me lo dices antes. = You tell me beforehand.
  This can sound like an instruction too, but grammatically it is the present tense, often used with an imperative feel in conversation.

The version with dímelo is the most straightforward and standard command.

Antes means before or beforehand.

In this sentence, it means:

• tell me in advance
• let me know beforehand

It does not need an extra word like de here, because it is being used on its own.

Compare:

• Avísame antes. = Let me know beforehand.
• antes de salir = before leaving

So:

• antes = beforehand
• antes de + infinitive/noun = before doing something

The comma separates the if-clause from the main clause:

• Si quieres cambiar la cita, = If you want to change the appointment,
• dímelo antes. = tell me beforehand.

This is very natural in Spanish writing. It helps show the two parts of the sentence clearly.

No. They are different words:

• si = if
• sí = yes

The accent mark changes the meaning.

So here:

• Si quieres cambiar la cita... = If you want to change the appointment...

Not:

• Yes you want...

It is informal Spanish, and it works in Spain very naturally.

The key informal features are:

• quieres = tú form
• dímelo = informal command to one person

This sentence is not exclusive to Spain; it would also be understood throughout the Spanish-speaking world. But since you asked about Spain, it is exactly the kind of sentence you would hear there in an everyday informal context.

If you wanted the plural informal form used in Spain with vosotros, you could say:

• Si queréis cambiar la cita, decídmelo antes.

Yes. Very often that is exactly what it means in practice.

Literally:

• cambiar = to change

But when talking about an appointment, English often prefers:

• change the appointment
• reschedule the appointment
• move the appointment

So depending on context, cambiar la cita may mean changing the time, day, or other details of the appointment.