Αν ο φορτιστής δεν λειτουργεί, τον αντικαθιστώ με άλλον.

Αν means if and introduces a condition. εάν is a slightly more formal/explicit variant of the same word. In everyday Greek, αν is extremely common.
Both can be used here: Αν/Εάν ο φορτιστής δεν λειτουργεί, ...

Greek normally uses the definite article where English often doesn’t, especially with specific, known items in context (like the charger you’re talking about).
You can drop it sometimes (especially in headlines/notes), but the natural full sentence is ο φορτιστής.

λειτουργεί is present tense (3rd person singular) of λειτουργώ: it works / it is working.
In a conditional like this, the present often expresses a general rule/habit: “If the charger doesn’t work (in general/whenever that happens) …”

δεν is the standard negation used with the indicative mood (like normal present/past statements): δεν λειτουργεί = it doesn’t work.
It comes directly before the verb (and any object pronouns that might be attached to the verb are placed between δεν and the verb, if present).

It separates the if-clause (the condition) from the main clause (the result), similar to English:
If X, (then) Y.
Greek often uses this comma in this structure.

τον is the weak (clitic) object pronoun meaning him/it (masculine accusative singular). Here it means it, referring to ο φορτιστής (the charger).
In Greek, these weak object pronouns usually come before the verb:
τον αντικαθιστώ = I replace it.

Yes. That would use the full noun instead of the pronoun:
Αν ο φορτιστής δεν λειτουργεί, αντικαθιστώ τον φορτιστή με άλλον.
Using τον avoids repeating φορτιστής and sounds more natural in flowing speech.

Greek often uses the present to express a general procedure/habit:
“If it doesn’t work, I replace it (that’s what I do).”
If you mean a specific future action (“If it doesn’t work, I’ll replace it”), you’d commonly use:
Αν ο φορτιστής δεν λειτουργεί, θα τον αντικαταστήσω. (perfective, more “I will replace it”)

με = with and άλλον = another (masculine accusative singular).
It means: with another one (another charger).
It’s masculine because it’s shorthand for άλλον (φορτιστή). If it were a neuter thing, you’d use άλλο.

Yes, that’s also correct and common:
Αν δεν λειτουργεί ο φορτιστής, τον αντικαθιστώ με άλλον.
Greek word order is flexible; moving δεν λειτουργεί earlier can sound slightly more “condition-first,” but both versions are natural.