Her father engaged her to him, then her father died and her paternal uncles want to change the agreement

Praise be to Allah.

Firstly: 

Before answering your question, it is essential to draw your attention to the fact that working in mixed workplaces with men and women together is not permissible, and it is one of the doors that lead to corruption, as its effects on society are obvious. 

We have mentioned the evidence for the prohibition on mixing in question no. 1200. 

The one who is faced with the problem of working in a mixed environment – if he cannot leave that job – must avoid looking at women and being alone with them and talking to them about things that do not have to do with work. 

Among the negative consequences of haraam mixing is what happens between men and non-mahram women whom they call “work colleagues”, such as haraam looking, talking and correspondence, and in many cases it leads to haraam relationships. 

Secondly: 

With regard to your question, it seems from your question that you did not do the marriage contract with this woman. Based on that, you are still a “stranger” (non-mahram) to her so it is not permissible for you to be alone with her or to talk to her too much, until the marriage contract is done between you. The marriage contract with a woman is not valid unless it is done in the presence of her wali (guardian). As her father has died, guardianship passes to her paternal grandfather. If there is no paternal grandfather, then it passes to one of her brothers. If she has no brother then guardianship passes to her paternal uncles. The wali does not have the right to prevent marriage without an acceptable shar’i reason. If he does prevent her, then guardianship passes from him to the next closest relative, then to the sharee’ah judge or one who is acting in his stead. It should also be noted that the maternal uncles cannot be walis of the woman. 

See the order of guardianship in the answer to question no. 2127. 

Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) said: If the woman likes a man and he is compatible with her, then her guardian, such as her brother then paternal uncles, must marry her to him. If they prevent her from getting married, then the next closest wali should give her in marriage, or the ruler, without (the wali’s) permission, according to scholarly consensus. The wali does not have the right to force her to marry someone she does not like, or to prevent her from marrying someone she does like if he is compatible, according to scholarly consensus. It is only people of ignorance and wrongdoing who force women into marriage or prevent them from marrying, who give their female relatives in marriage to people whom they choose for their own purposes, not the woman’s interests, and force them or embarrass them into doing that, and prevent them from marrying those they want out of enmity towards them or to serve some purpose. All of these are acts of Jaahiliyyah (ignorance), oppression and enmity, and are forbidden by Allaah and His Messenger. The Muslims are unanimously agreed that they are haraam. Allaah has enjoined women’s guardians to look at the woman’s interests, not their own whims and desires, like all other guardians and deputies who act on behalf of others. Their aim should be the interests of the one on whose behalf they are acting, not their own whims and desires. This comes under the heading of the trust (amaanah) that Allaah has commanded should be fulfilled, as He says (interpretation of the meaning):  “Verily, Allaah commands that you should render back the trusts to those, to whom they are due; and that when you judge between men, you judge with justice” [al-Nisa’ 4:58]. This is also part of sincerity that is required. The Prophet (peace and blessings of Allaah be upon him) said: “Religion is sincerity (naseehah)” three times. It was asked, “To whom, O Messenger of Allaah?” He said, “To Allaah, to His Book, to His Messenger, to the leaders of the Muslims and to their common folk.” 

Majmoo’ al-Fataawa (32/52, 53) 

But we advise you not to marry her without the agreement of her family; it is essential to gain their approval and win them over, so that you will not be a cause of breaking of family ties that may never be healed after that.

And Allaah knows best.