Сначала нужно сменить пароль.

Russian often leaves out the person when it is obvious or when the statement is meant generally.

Here, нужно + infinitive makes an impersonal expression:

• нужно сменить пароль = it is necessary to change the password
• depending on context, this can mean you need to change the password, we need to change the password, or one must change the password

So the sentence does not explicitly say who must do it.

Сначала means first, at first, or before anything else.

In this sentence, it means:

• first
• as the first step
• before doing the next thing

So Сначала нужно сменить пароль means something like First, you need to change the password.

Putting сначала first is a very natural way to show sequence in Russian. It sets the frame right away: this is the first action.

Russian word order is flexible, so you may also see:

• Нужно сначала сменить пароль

Both are correct. The version with сначала first is slightly more neutral for instructions.

Here нужно is being used in an impersonal predicative construction, not as an adjective agreeing with a noun.

Compare:

• Нужно сменить пароль. = It is necessary to change the password.
• Нужен пароль. = A password is needed.

So in your sentence, нужно does not describe пароль. It expresses necessity for the whole action сменить пароль.

This is about aspect, which is very important in Russian.

• сменить = perfective
• менять = imperfective

Сменить focuses on a completed result: the password gets changed. That is why it fits well in an instruction or required step.

So:

• нужно сменить пароль = you need to change the password successfully, as a completed action

If you used менять, it would sound more like a process, repeated action, or a general activity, which is less natural here.

Yes, изменить пароль is also possible and very common.

The difference is subtle:

• сменить пароль often means replace the old password with a new one
• изменить пароль means modify/change the password

In many real situations, both work. For passwords, both are used, but сменить пароль often sounds especially natural when the idea is switch to a new password.

It is in the accusative case, because it is the direct object of сменить.

The noun пароль is:

• masculine
• singular
• inanimate

For masculine inanimate nouns, the accusative singular usually looks the same as the nominative singular.

So:

• nominative: пароль
• accusative: пароль

That is why the form does not visibly change here.

It is neutral. It works well in:

• instructions
• technical help
• interface messages
• general advice

Because there is no explicit ты or вы, it avoids sounding too personal and is very common in written Russian, especially in manuals or system messages.

The main stresses are:

• снача́ла
• ну́жно
• смени́ть
• паро́ль

A rough pronunciation guide:

• sna-CHA-la NUZH-na smye-NEET pa-ROL'

A more careful note:

• нужно is pronounced roughly NUZH-na
• сменить has stress on the last syllable
• пароль has stress on the second syllable

Yes, depending on context.

Нужно usually means:

• need to
• have to
• it is necessary to

In English, the best translation can vary:

• First, you need to change the password
• First, you should change the password
• The first step is to change the password

The Russian sentence itself is neutral and practical, so the exact English wording depends on tone and situation.