At the very least you need to give the league notice by March 1st of the year prior to which you want a uniform change and must then obtain approval by December 1st of the year prior to the change. Here is the procedure to change uniforms:
NFL CONSTITUTION and BYLAWS ARTICLE XIX 19.9
(D) No club shall have the right to make changes in its club colors and/or in the designs of its team helmets or uniforms except in accordance with the following provisions:
(1) Absent specific extenuating circumstances as determined by the Commissioner, if a club desires to make any changes in club colors, uniform appearance, designs of team helmets, designs of team uniforms, trademarks, or trade names, it must give written notice and details thereof to the League on or before March 1 of the year prior to the year in which it wishes to change; must comply with the uniform change notification and approval timeline as established by the League office and amended from time to time; and further must obtain approval from the League pursuant to Section 19.9(D)(2) by December 1 of the year prior to the year in which it wishes to change; otherwise it shall have no right to make any change for the succeeding season.
(2) Despite the provisions of the foregoing sub-paragraph (1) of this Section 19.9(D) and despite the fact that notice was given of the proposed change in the colors and/or designs of team helmet and uniforms as above set out, should such change, in the opinion of the Commissioner, result in any conflict with the club colors and/or helmet and uniform designs of any other club, and the clubs involved are unable to agree upon a method of solving such conflict, the Commissioner shall have the right to designate the club colors and/or the designs of the team helmet or uniforms to be used by the affected clubs in such season.
2002 RESOLUTION G-3
Third, be it Further Resolved, that a Club may not change its regular home and away uniforms more than once every five NFL seasons, and may not change its third uniform design more than once every five NFL seasons, absent specific extenuating circumstances (e.g., Club ownership change or relocation) as determined by the Commissioner.
2003 RESOLUTION G-1
Whereas, pursuant to 2002 Resolution G-3, the Member Clubs may wear an approved third uniform design at one home game each season in connection with a special event, occasion or anniversary, provided that the Club gives notice to the League office by July 1 of the year in which the selected game is scheduled to be
played, the selected game does not conflict with a League event or initiative, and the event being commemorated is approved by the Commissioner (“the Third Uniform Design Program”);
Whereas, pursuant to Article 19.9 of the NFL Constitution and Bylaws, the home team has the option of deciding whether the visiting club shall wear white jerseys or the colors awarded to the visiting team under Section 19.9, and the Commissioner has authority to resolve any conflict or asserted conflict between the colors of the two clubs;
Whereas, the Member Clubs believe that it would be beneficial to authorize the Business Ventures Committee to oversee implementation of the Third Uniform Design Program; and
Whereas, developments in the NFL’s consumer products business make it desirable to expand the Third Uniform Design Program to allow a third uniform design at two regular season games, home or away.
Be it Resolved, that, subject to the conditions prescribed by 2002 Resolution G-3, a Club may wear an approved third uniform design at two regular season games, home or away; and
Further Resolved, that the Business Ventures Committee shall be authorized to oversee implementation of the Third Uniform Design Program.